Revert "Require Java 21 in plugin pom (#1286)"

[NotMyFault]

This reverts commit 7d6ddb2, but leaves the pom version incremement out.

Testing done

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

[jglick]

Reverts #1286, for the backlink.