[Forwardport] Add 2.9.1 to BWC and known versions (#8923)

[reta]

Forwardport of the #8923 to main

[kotwanikunal]

Thanks again @reta!

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❕
• TEST FAILURES:

      3 org.opensearch.snapshots.DedicatedClusterSnapshotRestoreIT.testIndexDeletionDuringSnapshotCreationInQueue

• URL: https://build.ci.opensearch.org/job/gradle-check/21100/
• CommitID: a511bf0
  Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[codecov]

Codecov Report

Merging #8929 (a511bf0) into main (99f28cb) will increase coverage by 0.04%.
Report is 1 commits behind head on main.
The diff coverage is 40.54%.

@@             Coverage Diff              @@
##               main    #8929      +/-   ##
============================================
+ Coverage     71.11%   71.15%   +0.04%     
- Complexity    57244    57256      +12     
============================================
  Files          4758     4758              
  Lines        269840   269863      +23     
  Branches      39480    39484       +4     
============================================
+ Hits         191897   192031     +134     
+ Misses        61786    61639     -147     
- Partials      16157    16193      +36     

Files Changed	Coverage Δ
...on/admin/cluster/RestRestoreRemoteStoreAction.java	30.00% <0.00%> (-3.34%)	⬇️
.../java/org/opensearch/snapshots/RestoreService.java	53.25% <0.00%> (-1.03%)	⬇️
...ibs/core/src/main/java/org/opensearch/Version.java	77.77% <100.00%> (+0.59%)	⬆️
...remotestore/restore/RestoreRemoteStoreRequest.java	74.13% <100.00%> (+4.13%)	⬆️
.../opensearch/cluster/routing/IndexRoutingTable.java	76.22% <100.00%> (+0.19%)	⬆️
...a/org/opensearch/cluster/routing/RoutingTable.java	94.37% <100.00%> (ø)

... and 459 files with indirect coverage changes