Update spotless to meet JDK-21 baseline

[reta]

Is your feature request related to a problem? Please provide an existing Issue # , or describe.
Add JDK-21 to Github actions

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Check List

• Backport Labels added.
• 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

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
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[reta]

@ansjcy purely formatting updates (spotless update is needed to work with JDK-21)

[ansjcy]

LGTM.

[reta]

@ansjcy we need to work on PowerMock removal, #529, it is not playing well with recent JDKs (and is dead essentially)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (344cae1) 74.41% compared to head (4041830) 74.42%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #533   +/-   ##
=========================================
  Coverage     74.41%   74.42%           
  Complexity     2666     2666           
=========================================
  Files           317      317           
  Lines         16377    16377           
  Branches       1282     1282           
=========================================
+ Hits          12187    12188    +1     
+ Misses         3675     3674    -1     
  Partials        515      515           

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[prudhvigodithi]

Looks like this is the change required for 2.12 builds to pass, opensearch-project/performance-analyzer#611, can we please get this in soon and backport to 2.x?

[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-533-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 3c92f12e3ef0789bc7c7bda2a28b85a350008e80
# Push it to GitHub
git push --set-upstream origin backport/backport-533-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-533-to-2.x.