Remove Identity FeatureFlag

[cwperks]

Description

This PR removes the identity feature flag.

The PR is a follow-up to 2 previous PRs:

• Add runAs to Subject interface and introduce IdentityAwarePlugin extension point #14630
• Remove identity-related feature flagged code from the RestController #15430

This PR is a prerequisite for opensearch-project/opensearch-plugins#238 to begin creating issues on individual plugin repos to adopt the new mechanism for system index interaction

There are 2 additional open PRs (1 in core repo and 1 in security repo) that need to be resolved before plugins start to adopt the new mechanism.

1. Allow Plugins to request to perform cluster actions and index actions with their assigned PluginSubject and prompt on install #15778
2. Implement new extension points in IdentityPlugin and add ContextProvidingPluginSubject security#4665

Related Issues

Resolves opensearch-project/security#4439

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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 result for 1a07f98: FAILURE

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 result for 1a07f98: FAILURE

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 result for 1a07f98: SUCCESS

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.92%. Comparing base (a767e92) to head (8d18592).
Report is 4 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #16024      +/-   ##
============================================
- Coverage     71.98%   71.92%   -0.07%     
- Complexity    64542    64550       +8     
============================================
  Files          5288     5289       +1     
  Lines        301474   301503      +29     
  Branches      43552    43557       +5     
============================================
- Hits         217024   216861     -163     
- Misses        66657    66851     +194     
+ Partials      17793    17791       -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[reta]

Thanks @cwperks !

[cwperks]

Made a small change in the CHANGELOG notes to move another entry from the Added section to the Changed section.

[github-actions]

❕ Gradle check result for 8d18592: UNSTABLE

Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[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:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/OpenSearch/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/OpenSearch/backport-2.x
# Create a new branch
git switch --create backport/backport-16024-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 7b248efc5ab50573ed25a9eaac5d44c9b75c243a
# Push it to GitHub
git push --set-upstream origin backport/backport-16024-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/OpenSearch/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-16024-to-2.x.

[reta]

@cwperks could you please backport to 2.x manually? thank you

[cwperks]

Opened a manual backport: #16157