Add release notes v2.8.0

[cwperks]

Description

This PR adds release notes for the v2.8.0 release.

Related Issues

#7664

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: UNSTABLE ❕
• TEST FAILURES:

      1 org.opensearch.search.backpressure.SearchBackpressureIT.testSearchShardTaskCancellationWithHighCpu
      1 org.opensearch.remotestore.RemoteStoreRefreshListenerIT.testRemoteRefreshRetryOnFailure
      1 org.opensearch.index.ShardIndexingPressureIT.testShardIndexingPressureTrackingDuringBulkWrites

• URL: https://build.ci.opensearch.org/job/gradle-check/16547/
• CommitID: f910586
  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 #7825 (f910586) into main (a201400) will decrease coverage by 0.16%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##               main    #7825      +/-   ##
============================================
- Coverage     70.92%   70.77%   -0.16%     
+ Complexity    56276    56199      -77     
============================================
  Files          4683     4683              
  Lines        266113   266113              
  Branches      39075    39075              
============================================
- Hits         188749   188338     -411     
- Misses        61402    61845     +443     
+ Partials      15962    15930      -32     

see 460 files with indirect coverage changes

[andrross]

We're still accepting 2.8 changes through the end of the day, so lets hold off on merging this until tomorrow so that we can bring in any last minute 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-7825-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 20387c048bf9ccc6376adfec84af26261f9754b1
# Push it to GitHub
git push --set-upstream origin backport/backport-7825-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-7825-to-2.x.

[opensearch-trigger-bot]

The backport to 2.8 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-2.8 2.8
# Navigate to the new working tree
pushd ../.worktrees/backport-2.8
# Create a new branch
git switch --create backport/backport-7825-to-2.8
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 20387c048bf9ccc6376adfec84af26261f9754b1
# Push it to GitHub
git push --set-upstream origin backport/backport-7825-to-2.8
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/backport-2.8

Then, create a pull request where the base branch is 2.8 and the compare/head branch is backport/backport-7825-to-2.8.