[JENKINS-57683] Fix class cast exception scanning multibranch pipeline

[MarkEWaite]

JENKINS-57683 - Fix class cast exception scanning multibranch pipeline

PruneStaleBranchTrait implementation did not handle SCMSource which was not a GitSCMSource.
Added a failing test, then fixed the failing test.

Checklist

• I have read the CONTRIBUTING doc
• I have referenced the Jira issue related to my changes in one or more commit messages
• I have added tests that verify my changes
• Unit tests pass locally with my changes
• I have added documentation as necessary
• No Javadoc warnings were introduced with my changes
• No spotbugs warnings were introduced with my changes
• I have interactively tested my changes (I won't merge this until I've tested interactively)
• Any dependent changes have been merged and published in upstream modules (like git-client-plugin)

Types of changes

• Bug fix (non-breaking change which fixes an issue)