Set Java version to 21 for use in bwc tests (#18058)

My understanding is that BwcSetupExtension.java uses this Java version
to compile this code when another branch checks out this branch to use
in backward compatibility tests. This needs to be JDK21 in order for
this code to compile correctly.

Signed-off-by: Andrew Ross <[email protected]>

Author: andrross

.ci/java-versions.properties

@@ -14,7 +14,7 @@
 # are 'java' or 'openjdk' followed by the major release number.
 
 # See please https://docs.gradle.org/8.10/userguide/upgrading_version_8.html#minimum_daemon_jvm_version
-OPENSEARCH_BUILD_JAVA=openjdk17
-OPENSEARCH_RUNTIME_JAVA=java11
+OPENSEARCH_BUILD_JAVA=openjdk21
+OPENSEARCH_RUNTIME_JAVA=java21
 GRADLE_TASK=build
 GRADLE_EXTRA_ARGS=