Chore: Scope DependencyScanning to gemnasium

What does this MR do?

Disables gemnasium-maven which is incorrectly triggered due to test fixtures. Fixes failures such as https://gitlab.com/gitlab-org/security-products/analyzers/spotbugs/-/jobs/737116751

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• [-] 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 (also in CE)
  • Job definition example
  • CI Templates (to be removed)
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer