Require Jenkins 2.387.3 or newer

[MarkEWaite]

Require Jenkins 2.387.3 or newer

Includes other items of cleanup

• Check dependencies monthly
• Remove old changelog
• Remove contributors file - use git instead
• Minor docs cleanup
• Remove trailing spaces

Testing done

Confirmed that automated tests pass.

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 demonstrates feature works or fixes the issue