Bump 2.x to 2.9.0

[reta]

Description

Bump 2.x to 2.9.0, we already dropped 2.8.0 RC builds.

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/16650/
• CommitID: e26a3b3

[codecov]

Codecov Report

Merging #7864 (e26a3b3) into 2.x (f032091) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                2.x    #7864      +/-   ##
============================================
+ Coverage     70.43%   70.49%   +0.06%     
- Complexity    56345    56373      +28     
============================================
  Files          4665     4665              
  Lines        267674   267675       +1     
  Branches      39634    39634              
============================================
+ Hits         188531   188708     +177     
+ Misses        62837    62666     -171     
+ Partials      16306    16301       -5     

Impacted Files	Coverage Δ
server/src/main/java/org/opensearch/Version.java	83.54% <100.00%> (+0.07%)	⬆️

... and 445 files with indirect coverage changes