Simplify Jenkinsfile to match other plugins and archetype

Most plugins only test Java 25 and Java 21, since we've never detected an issue that was specific to Java 17 generating the byte code.

Simplify Jenkinsfile to match other plugins and archetype

Testing done

Test on ci.jenkins.io

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