Update ProblemList_openjdk21-openj9.txt to consolidate issues

Consolidating issue #1596 with #923

Author: dennycjacob

openjdk/excludes/ProblemList_openjdk21-openj9.txt

@@ -228,7 +228,7 @@ java/net/MulticastSocket/MulticastAddresses.java https://github.ibm.com/runtimes
 java/net/MulticastSocket/NoSetNetworkInterface.java https://github.ibm.com/runtimes/backlog/issues/655 windows-all
 java/net/MulticastSocket/Promiscuous.java https://github.ibm.com/runtimes/backlog/issues/923 linux-s390x,aix-all
 java/net/MulticastSocket/PromiscuousIPv6.java https://github.com/adoptium/infrastructure/issues/699 linux-s390x,macosx-all
-java/net/MulticastSocket/SetLoopbackMode.java https://github.ibm.com/runtimes/backlog/issues/1596 aix-all
+java/net/MulticastSocket/SetLoopbackMode.java https://github.ibm.com/runtimes/backlog/issues/923 aix-all
 java/net/MulticastSocket/SetLoopbackModeIPv4.java https://github.ibm.com/runtimes/backlog/issues/707 linux-s390x,macosx-all
 java/net/MulticastSocket/SetOutgoingIf.java https://github.ibm.com/runtimes/backlog/issues/715 macosx-all
 java/net/MulticastSocket/Test.java https://github.ibm.com/runtimes/backlog/issues/923 linux-s390x,aix-all