Create 2.9 release notes and clear out changelog

[andrross]

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.

[reta]

Thanks @andrross !

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/20127/
• CommitID: 1113fe6
  Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green.
  Is the failure a flaky test unrelated to your change?

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❕
• TEST FAILURES:

      1 org.opensearch.snapshots.DedicatedClusterSnapshotRestoreIT.testIndexDeletionDuringSnapshotCreationInQueue
      1 org.opensearch.remotestore.multipart.RemoteStoreMultipartIT.testStaleCommitDeletionWithoutInvokeFlush
      1 org.opensearch.remotestore.SegmentReplicationUsingRemoteStoreIT.testDropPrimaryDuringReplication

• URL: https://build.ci.opensearch.org/job/gradle-check/20129/
• CommitID: fb964d5
  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 #8682 (fb964d5) into main (064f265) will increase coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main    #8682   +/-   ##
=========================================
  Coverage     70.87%   70.88%           
+ Complexity    57201    57198    -3     
=========================================
  Files          4771     4771           
  Lines        270312   270312           
  Branches      39505    39505           
=========================================
+ Hits         191590   191611   +21     
- Misses        62619    62623    +4     
+ Partials      16103    16078   -25     

see 456 files with indirect coverage changes

[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/backport-2.x
# Create a new branch
git switch --create backport/backport-8682-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 3254efd7decd0d9a97caa69982286b1b90198585
# Push it to GitHub
git push --set-upstream origin backport/backport-8682-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-8682-to-2.x.

[opensearch-trigger-bot]

The backport to 2.9 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/backport-2.9 2.9
# Navigate to the new working tree
pushd ../.worktrees/backport-2.9
# Create a new branch
git switch --create backport/backport-8682-to-2.9
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 3254efd7decd0d9a97caa69982286b1b90198585
# Push it to GitHub
git push --set-upstream origin backport/backport-8682-to-2.9
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/backport-2.9

Then, create a pull request where the base branch is 2.9 and the compare/head branch is backport/backport-8682-to-2.9.