Modernize to Jenkins 2.479 and Jakarta EE 9

[mtughan]

• Adapter methods are added for old overrides.
• Switch to JUnit 5 for tests

Required for the Scriptler plugin to modernize as well without breaking SSH compatibility.

Testing done

Unit tests updated and run.

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

[mtughan]

Trying to figure out why SpotBugs is errantly flagging a null issue where one doesn’t exist and why it’s not suppressing.

[mtughan]

Turns out that this content type has been missing for who knows how long, so CSRF protection would have blocked any Git fetches to a repository. Given that, I'm going to probably add functionality to the Scriptler plugin to enable/disable the Git repository (defaulting to disabled) and see what happens. I'm suspecting that no one uses it.

SSH access still works, but I feel like that would be less commonly used because you'd also have to enable the SSH server within Jenkins first.

[mtughan]

Just read the comments in #112 about HTTP access. Without knowing more about HTTP access, I'm removing this part from the commit and it can be dealt with separately.

[mtughan]

@jenkinsci/git-server-plugin-developers, please review when able.