Make gemnasium-maven search a single dir

What does this MR do?

• Change finder.PresetGemnasiumMaven to SearchSingleDir, in alignment with gemnasium-maven!131 (merged).
• Remove finder.SearchSingleTree for it's no longer needed.

This doesn't change the behavior of gemnasium, and so there's no changelog entry for this.

What are the relevant issue numbers?

gitlab-org/gitlab#324626 (closed)

Does this MR meet the acceptance criteria?

• [N/A] Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer