Teardown/Archive legacy Gemnasium infra

Linked gitlab-org issue: gitlab-org/gitlab#14692 (comment 1065011193)

Implementation plan

• Verify (inactivity?) activity (inbound traffic to the WEB/API endpoints)
• Do a backup/archive of the whole PGSQL DB for the purpose of restoration if there is some immediate problem that arises after the decom. Delete the backup after 30 days to ensure no conflict with our privacy policy.
• Delete the whole cluster (no need to remove resources one by one)
• Delete deps.sec.gitlab.com from the DNS
• Archive https://gitlab.com/gitlab-org/security-products/gemnasium/gcp-config.

SRE Support Needed Infra will be needed to help with this work.