FINERACT-2094: Update to Java 21

modernize locale instantiations (fixes an error found by the modernizer plugin) - use Locale.of() instead of new Locale()

use recommended gradle toolchain syntax to specify Java class file format version see https://docs.gradle.org/current/userguide/java_plugin.html#toolchain_and_compatibility

mark html5 for later fix (it's the default)

use dummy byte arrays to work around mockito exception

```
org.mockito.exceptions.base.MockitoException:
Mockito cannot mock this class: class java.nio.ByteBuffer.

If you're not sure why you're getting this error, please open an issue on GitHub.

Java               : 21
JVM vendor name    : Ubuntu
JVM vendor version : 21.0.6+7-Ubuntu-122.04.1
JVM name           : OpenJDK 64-Bit Server VM
JVM version        : 21.0.6+7-Ubuntu-122.04.1
JVM info           : mixed mode, sharing
OS name            : Linux
OS version         : 5.15.0-136-generic

You are seeing this disclaimer because Mockito is configured to create inlined mocks.
You can learn about inline mocks and their limitations under item #39 of the Mockito class javadoc.

Underlying exception : org.mockito.exceptions.base.MockitoException: Unsupported settings with this type 'java.nio.ByteBuffer'
	at app//org.apache.fineract.infrastructure.event.external.jobs.SendAsynchronousEventsTaskletTest.givenBatchSize2WhenEventSendFailsThenExecutionStops(SendAsynchronousEventsTaskletTest.java:170)
	at java.base@21.0.6/java.lang.reflect.Method.invoke(Method.java:580)
	at java.base@21.0.6/java.util.ArrayList.forEach(ArrayList.java:1596)
	at java.base@21.0.6/java.util.ArrayList.forEach(ArrayList.java:1596)
Caused by: org.mockito.exceptions.base.MockitoException: Unsupported settings with this type 'java.nio.ByteBuffer'
	at app//net.bytebuddy.TypeCache.findOrInsert(TypeCache.java:168)
	at app//net.bytebuddy.TypeCache$WithInlineExpunction.findOrInsert(TypeCache.java:399)
	at app//net.bytebuddy.TypeCache.findOrInsert(TypeCache.java:190)
	at app//net.bytebuddy.TypeCache$WithInlineExpunction.findOrInsert(TypeCache.java:410)
	... 4 more
```

see also: https://stackoverflow.com/questions/47046996/java-lang-unsupportedoperationexception-when-mocking-java-nio-bytebuffer-class

wrap byte arrays with ByteBuffer

fix:

```
/home/adamm/git/apache/fineract/fineract-provider/src/test/java/org/apache/fineract/infrastructure/event/external/jobs/SendAsynchronousEventsTaskletTest.java:148: error: incompatible types: byte[] cannot be converted to ByteBuffer
                "anidempotencyKey", "aSchema", "dummy".getBytes());
                                                               ^
/home/adamm/git/apache/fineract/fineract-provider/src/test/java/org/apache/fineract/infrastructure/event/external/jobs/SendAsynchronousEventsTaskletTest.java:170: error: incompatible types: byte[] cannot be converted to ByteBuffer
                "anidempotencyKey", "aSchema", "dummy".getBytes());
                                                               ^
/home/adamm/git/apache/fineract/fineract-provider/src/test/java/org/apache/fineract/infrastructure/event/external/jobs/SendAsynchronousEventsTaskletTest.java:189: error: incompatible types: byte[] cannot be converted to ByteBuffer
                "anidempotencyKey", "aSchema", "dummy".getBytes());
                                                               ^
/home/adamm/git/apache/fineract/fineract-provider/src/test/java/org/apache/fineract/infrastructure/event/external/jobs/SendAsynchronousEventsTaskletTest.java:210: error: incompatible types: byte[] cannot be converted to ByteBuffer
                "anidempotencyKey", "aSchema", "dummy".getBytes());
```

...and use StandardCharsets with getBytes

fineract-core/src/main/java/org/apache/fineract/infrastructure/core/domain/AbstractAuditableWithUTCDateTimeCustom.java

@@ -53,19 +53,19 @@ public abstract class AbstractAuditableWithUTCDateTimeCustom<T extends Serializa
     private static final long serialVersionUID = 141481953116476081L;
 
     @Column(name = CREATED_BY_DB_FIELD, updatable = false, nullable = false)
-    @Setter(onMethod = @__(@Override))
+    @Setter(onMethod_ = @Override)
     private Long createdBy;
 
     @Column(name = CREATED_DATE_DB_FIELD, updatable = false, nullable = false)
-    @Setter(onMethod = @__(@Override))
+    @Setter(onMethod_ = @Override)
     private OffsetDateTime createdDate;
 
     @Column(name = LAST_MODIFIED_BY_DB_FIELD, nullable = false)
-    @Setter(onMethod = @__(@Override))
+    @Setter(onMethod_ = @Override)
     private Long lastModifiedBy;
 
     @Column(name = LAST_MODIFIED_DATE_DB_FIELD, nullable = false)
-    @Setter(onMethod = @__(@Override))
+    @Setter(onMethod_ = @Override)
     private OffsetDateTime lastModifiedDate;
 
     @Override

fineract-core/src/main/java/org/apache/fineract/infrastructure/core/domain/AbstractPersistableCustom.java

@@ -54,12 +54,12 @@ public abstract class AbstractPersistableCustom<T extends Serializable> implemen
 
     @Id
     @GeneratedValue(strategy = GenerationType.IDENTITY)
-    @Getter(onMethod = @__(@Override))
+    @Getter(onMethod_ = @Override)
     private T id;
 
     @Transient
     @Setter(value = AccessLevel.NONE)
-    @Getter(onMethod = @__(@Override))
+    @Getter(onMethod_ = @Override)
     private boolean isNew = true;
 
     @PrePersist

fineract-core/src/main/java/org/apache/fineract/infrastructure/core/serialization/GoogleGsonSerializerHelper.java

@@ -105,7 +105,7 @@ public final class GoogleGsonSerializerHelper {
     public static void registerTypeAdapters(final GsonBuilder builder) {
         builder.registerTypeAdapter(java.util.Date.class, new DateAdapter());
         builder.registerTypeAdapter(LocalDate.class, new LocalDateAdapter());
-        // NOTE: was missing, necessary for GSON serialization with JDK 17's restrictive module access
+        // NOTE: was missing, necessary for GSON serialization with JDK 21's restrictive module access
         builder.registerTypeAdapter(LocalTime.class, new LocalTimeAdapter());
         builder.registerTypeAdapter(ZonedDateTime.class, new JodaDateTimeAdapter());
         builder.registerTypeAdapter(MonthDay.class, new JodaMonthDayAdapter());

fineract-core/src/main/java/org/apache/fineract/infrastructure/core/serialization/JsonParserHelper.java

@@ -819,7 +819,7 @@ public class JsonParserHelper {
                     dataValidationErrors);
         }
 
-        return new Locale(languageCode.toLowerCase(), courntryCode.toUpperCase(), variantCode);
+        return Locale.of(languageCode.toLowerCase(), courntryCode.toUpperCase(), variantCode);
     }
 
     private Locale extractLocaleValue(final JsonObject object) {

fineract-core/src/main/java/org/apache/fineract/infrastructure/security/domain/BasicPasswordEncodablePlatformUser.java

@@ -32,9 +32,9 @@ public class BasicPasswordEncodablePlatformUser implements PlatformUser {
 
     @Getter
     private Long id;
-    @Getter(onMethod = @__(@Override))
+    @Getter(onMethod_ = @Override)
     private String username;
-    @Getter(onMethod = @__(@Override))
+    @Getter(onMethod_ = @Override)
     private String password;
 
     @Override