Added support for jdk-21

[martin-gaievski]

Description

Adding jdk-21 support:

• upgrade gradle to 8.4
• added jdk-21 to CI configuration

This is a requirement for 2.12 release, core build system already switched to jdk-21 as default one opensearch-project/OpenSearch#11003. I've created a PR to fix this opensearch-project/opensearch-build#4268, but the build for plugin is failing in distribution build CI

Some additional changes:

• upgrade of spotless plugin, it has new standards which affect many classes. All changes in java files are related to compliance with that new version of spotless.
• increase max JVM heap to 4Gb as otherwise ml-commons is failing with Circuit Breaker exception

Check List

• New functionality includes testing.
  • All tests pass
• Commits are signed as per the DCO using --signoff

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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c338b94) 84.55% compared to head (1b52576) 84.55%.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #500   +/-   ##
=========================================
  Coverage     84.55%   84.55%           
  Complexity      532      532           
=========================================
  Files            40       40           
  Lines          1547     1547           
  Branches        244      244           
=========================================
  Hits           1308     1308           
  Misses          128      128           
  Partials        111      111           

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

[martin-gaievski]

I've up versioned all libraries that we're referencing directly. For some libraries later version is available, but we're blocked on upgrade due to other packages that we depend on. Unless that's fixed the Mend Security Check will stay there

[opensearch-trigger-bot]

The backport to 2.x 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.x 2.x
# Navigate to the new working tree
cd .worktrees/backport-2.x
# Create a new branch
git switch --create backport/backport-500-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 5daddfd2f10486ad4688742aa26db82461b6a73c
# Push it to GitHub
git push --set-upstream origin backport/backport-500-to-2.x
# Go back to the original working tree
cd ../..
# 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-500-to-2.x.