Require Jenkins 2.479.1 or newer and Jakarta EE 9

Jenkins 2.479.1 provides Jakarta EE 9, Eclipse Jetty 12, Spring Security 6, and Java 17.

• Update plugin pom and baseline
• Remove deprecations
• Use Java 17 language features
• Minor cleanup

Testing done

mvn clean verify

Submitter checklist

• Make sure you are opening from atopic/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 demonstrates feature works or fixes the issue