Exclude gemnasium-db from automatic review assignment

What does this MR do and why?

Describe in detail what your merge request does and why.

The https://gitlab.com/gitlab-org/security-products/gemnasium-db project has a rotating engineer from groupvulnerability research who is responsible for reviewing and merging all of the MRs. Since this person will review any community contributions as part of this process, it's not necessary to automatically assign an MR coach when these are ready for review.

Expected impact & dry-runs

These are strongly recommended to assist reviewers and reduce the time to merge your change.

See https://gitlab.com/gitlab-org/quality/triage-ops/-/tree/master/doc/scheduled#testing-with-a-dry-run on how to perform dry-runs.

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations => gitlab-com/www-gitlab-com!113418 (merged)
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review