Restore testing againt Java 21 on CI.

Our release tools pushed out an update to our pipeline.properties. Because Java 21 hasn't been updated (yet) there, it overwrote our testing against Java 21. This should reinstate things until we can roll this out everywhere.

See #2993, #2950

Author: gregturn

ci/pipeline.properties

@@ -1,10 +1,10 @@
 # Java versions
 java.main.tag=17.0.7_7-jdk-focal
-java.next.tag=20-jdk-jammy
+java.next.tag=21-jdk-bullseye
 
 # Docker container images - standard
 docker.java.main.image=harbor-repo.vmware.com/dockerhub-proxy-cache/library/eclipse-temurin:${java.main.tag}
-docker.java.next.image=harbor-repo.vmware.com/dockerhub-proxy-cache/library/eclipse-temurin:${java.next.tag}
+docker.java.next.image=harbor-repo.vmware.com/dockerhub-proxy-cache/library/openjdk:${java.next.tag}
 
 # Supported versions of MongoDB
 docker.mongodb.4.4.version=4.4.22