[JENKINS-43052] Warn in Pipeline build log if a deprecated extension is used

[Nimbusprogrammer]

Implements Pipeline-only DEPRECATED warnings when deprecated GitSCM extensions are used, as described in JENKINS-43052.

The warning is written only to Pipeline build logs and does not alter behavior. Freestyle jobs are unaffected. Where applicable, the warning suggests the preferred Pipeline-native alternative.

Testing done

• Added JUnit 5 Workflow tests using JenkinsRule and @WithGitSampleRepo to verify that DEPRECATED warnings are logged for Pipeline jobs using deprecated extensions.
• Verified locally that Pipeline jobs log the warning and Freestyle jobs do not.

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 demonstrate the feature works or the issue is fixed

Jira: https://issues.jenkins.io/browse/JENKINS-43052

[MarkEWaite]

@Nimbusprogrammer you need to resolve the spotbugs failure before I'm willing to review the proposed changes. Having spotbugs warnings says that you need to follow the instructions in the contributing guide and run mvn clean verify locally. That will fail due to the spotbugs warning and you can fix it locally.

Please close the preceding pull request if you are replacing it with this one.