Update CI to use jdk-21

[ryanbogan]

Description

This updates the CI system to use jdk-21, which is latest LTS supported version. Coming from https://github.com/opensearch-project/OpenSearch/issues/10334

Check List

• [X] 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. For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (70f6751) 89.03% compared to head (11fabde) 89.14%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #599 +/- ## ============================================ + Coverage 89.03% 89.14% +0.10% - Complexity 767 768 +1 ============================================ Files 93 93 Lines 2746 2746 Branches 223 223 ============================================ + Hits 2445 2448 +3 + Misses 221 219 -2 + Partials 80 79 -1 ```

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[opensearch-trigger-bot[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-599-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 34aec653efb7f17a68c89da9fb7ce53b25b896d0
# Push it to GitHub
git push --set-upstream origin backport/backport-599-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-599-to-2.x.