Refresh Plugin for 2.361.x LTS and Java 11 baseline

[strangelookingnerd]

This PR includes:

• Upgrade to 2.361.x LTS and Java 11 baseline
• Some minor code cleanup (unused imports, unnecessary generics, removed deprecations)
• Supersedes #10

I wanted to migrate to JUnit5 as well but it seems like RunListenerTest relies on some JUnit4 injections that are not yet upgraded, so I let it be for now.

• [x] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• [x] Ensure that the pull request title represents the desired changelog entry
• [x] Please describe what you did
• [x] Link to relevant issues in GitHub or Jira
• [x] Link to relevant pull requests, esp. upstream and downstream changes
• [x] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[strangelookingnerd]

Seems like ci.jenkins.io does not like me changing the Jenkinsfile 🤷🏼

10:04:48  Connecting to https://api.github.com/ to check permissions of obtain list of strangelookingnerd for jenkinsci/generic-event-plugin
10:04:48  Loading trusted files from base branch main at 080de468d609debd3a9995eab2176aa8bd998b27 rather than 3803c8a5f3fc4bfe48492abe05e65c33d07ca0d5
10:04:48  Obtained Jenkinsfile from 080de468d609debd3a9995eab2176aa8bd998b27
10:04:49  ‘Jenkinsfile’ has been modified in an untrusted revision

However the Java CI check succeeds and I verified it locally as well.