Update CI to test on JDK 21

[dbwiddis]

Description

Switches testing from JDK 20 (EOL) to JDK 21 (LTS)

Matches OpenSearch JDK testing: https://github.com/opensearch-project/OpenSearch/blob/3c14cc187525fde6cc24fd5e3acec18c44ce9a1d/.github/workflows/precommit.yml#L10

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[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/flow-framework/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/flow-framework/backport-2.x
# Create a new branch
git switch --create backport/backport-267-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 287f48090000bb5d8daa1a7d8c5ed2fbc365fd35
# Push it to GitHub
git push --set-upstream origin backport/backport-267-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/flow-framework/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-267-to-2.x.

[dbwiddis]

To backport manually, run these commands in your terminal:

Intentionally not backporting; we were only running on JDK11 so the 20->21 change doesn't apply