ZOOKEEPER-5043: Disable fallback to DIGEST-MD5 in SaslServer when FIPS mode is set

[PDavid]

Reviewers: anmolnar, meszibalu
Author: PDavid
Closes #2381 from PDavid/ZOOKEEPER-5043-disable-DIGEST-MD5

(cherry picked from commit 972e71c)

[PDavid]

This is a backport of #2381 to branch-3.9.

The cherry-pick did not apply cleanly, had to resolve a small conflict in zookeeper-client/zookeeper-client-c/tests/zkServer.sh.

[PDavid]

Some tests failed in the Jenkins PR build. Locally they pass for me:

INFO] -------------------------------------------------------
[INFO]  T E S T S
[INFO] -------------------------------------------------------
[INFO] Running org.apache.zookeeper.server.quorum.StandaloneDisabledTest
[INFO] Running org.apache.zookeeper.server.quorum.DIFFSyncTest
[INFO] Running org.apache.zookeeper.server.quorum.CloseSessionTxnTest
[INFO] Running org.apache.zookeeper.server.quorum.QuorumSSLTest
[INFO] Tests run: 2, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 12.997 s - in org.apache.zookeeper.server.quorum.DIFFSyncTest
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 27.959 s - in org.apache.zookeeper.server.quorum.CloseSessionTxnTest
[INFO] Tests run: 2, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 88.404 s - in org.apache.zookeeper.server.quorum.StandaloneDisabledTest
[INFO] Tests run: 22, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 561.286 s - in org.apache.zookeeper.server.quorum.QuorumSSLTest
[INFO] 
[INFO] Results:
[INFO] 
[INFO] Tests run: 27, Failures: 0, Errors: 0, Skipped: 0
[INFO] 
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time:  09:29 min
[INFO] Finished at: 2026-05-11T15:51:04+02:00
[INFO] ------------------------------------------------------------------------

[anmolnar]

One quick question.