Increment version to 2.9.1

[vibrantvarun]

I've noticed that a new tag 2.9.0 was pushed, and incremented the version from 2.9.0 to 2.9.1.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❕
• TEST FAILURES:

      1 org.opensearch.index.translog.RemoteFSTranslogTests.testConcurrentWriteViewsAndSnapshot

• URL: https://build.ci.opensearch.org/job/gradle-check/21011/
• CommitID: 9729654
  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 #8897 (9729654) into 2.9 (3a7c95a) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                2.9    #8897      +/-   ##
============================================
+ Coverage     70.60%   70.68%   +0.07%     
- Complexity    57113    57128      +15     
============================================
  Files          4746     4746              
  Lines        270722   270723       +1     
  Branches      39952    39952              
============================================
+ Hits         191153   191356     +203     
+ Misses        63115    62904     -211     
- Partials      16454    16463       +9     

Files Changed	Coverage Δ
...ibs/core/src/main/java/org/opensearch/Version.java	82.74% <100.00%> (+0.07%)	⬆️

... and 436 files with indirect coverage changes