Switch main/3.x to use JDK21 LTS version

Signed-off-by: Peter Zhu <zhujiaxi@amazon.com>

Author: peterzhuamazon

buildSrc/src/main/java/org/opensearch/gradle/test/DistroTestPlugin.java

@@ -77,9 +77,9 @@
 import java.util.stream.Stream;
 
 public class DistroTestPlugin implements Plugin<Project> {
-    private static final String SYSTEM_JDK_VERSION = "23.0.2+7";
+    private static final String SYSTEM_JDK_VERSION = "21.0.6+7";
     private static final String SYSTEM_JDK_VENDOR = "adoptium";
-    private static final String GRADLE_JDK_VERSION = "23.0.2+7";
+    private static final String GRADLE_JDK_VERSION = "21.0.6+7";
     private static final String GRADLE_JDK_VENDOR = "adoptium";
 
     // all distributions used by distro tests. this is temporary until tests are per distribution

gradle/libs.versions.toml

@@ -3,7 +3,7 @@ opensearch        = "3.0.0"
 lucene            = "10.1.0"
 
 bundled_jdk_vendor = "adoptium"
-bundled_jdk = "23.0.2+7"
+bundled_jdk = "21.0.6+7"
 
 # optional dependencies
 spatial4j         = "0.7"