Add custom ruby-advisory-db

What does this MR do?

This adds 2 new variables to bundler-audit which allow users to customize the ruby-advisory-db used by the bundler-audit gem.

What are the relevant issue numbers?

gitlab-org/gitlab#37722 (closed)

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
  • [-] Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• [-] Security reports checked/validated by reviewer