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

custom/docker/build.gradle

@@ -24,7 +24,7 @@ apply from: "${rootDir}/buildSrc/src/main/groovy/org.apache.fineract.dependencie
 
 jib {
     from {
-        image = 'azul/zulu-openjdk-alpine:17'
+        image = 'azul/zulu-openjdk-alpine:21'
         platforms {
             platform {
                 architecture = System.getProperty("os.arch").equals("aarch64")?"arm64":"amd64"