Use jenkins.baseline to reduce bom update mistakes

[strangelookingnerd]

Use jenkins.baseline in pom.xml

This change is proposed by the plugin archetype and makes maintenance of jenkins.version and bom a little easier.

Testing done

None. Rely on ci.jenkins.io to test it.

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

[strangelookingnerd]

@jenkinsci/unity3d-plugin-developers Kindly requesting a review.

[BobDu]

LGTM