Increasing Spotbugs level and fixing Spotbugs issues

[aneveux]

Hello folks 👋

I'm currently following the Improve a Plugin documentation for
Jenkins plugins modernization, and noticed we could add more Spotbugs checks in this plugin.

So here's a PR following the documentation recommendation by increasing the Spotbugs checks, as well as fixing the
reported issues.

Let me know if that sounds interesting to you or if any other modifications are needed.

Thanks a lot!

• 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 // I do not have a Jira issue for that. Should I create one?
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[jglick]

Thanks!

[jglick]

Unfortunate. Guess I need to set up Spotless to enforce simple lexicographical import sorting.