Raise the minimum Java version to 21. (#765)

See also Mockito documentation: [Explicitly setting up instrumentation for inline mocking (Java 21+)](https://javadoc.io/doc/org.mockito/mockito-core/latest/org.mockito/org/mockito/Mockito.html#0.3)

--------

* JDK 21:

WARNING: A Java agent has been loaded dynamically (.../byte-buddy-agent-1.12.19.jar)
WARNING: If a serviceability tool is in use, please run with -XX:+EnableDynamicAgentLoading to hide this warning
WARNING: If a serviceability tool is not in use, please run with -Djdk.instrument.traceUsage for more information
WARNING: Dynamic loading of agents will be disallowed by default in a future release

Author: blackwinter

build.gradle

@@ -37,7 +37,7 @@ subprojects {
       'everit':           '1.14.6',
       'guava':            '32.0.1-jre',
       'jackson':          '2.21.4',
-      'jdk':              '11',
+      'jdk':              '21',
       'jdom':             '2.0.6.1',
       'jena':             '4.10.0',
       'jndi':             '0.11.4.1',
@@ -195,6 +195,9 @@ subprojects {
       showStandardStreams = true
       exceptionFormat = 'full'
     }
+
+    // Required for ByteBuddy agent, used by EqualsVerifier and Mockito
+    jvmArgs += ['-XX:+EnableDynamicAgentLoading']
   }
 
   repositories {