Resolve incorrect connection usage raised in spec/lib/gitlab/background_migration/backfill_vulnerability_finding_links_project_id_spec.rb

This migration has table interactions crossing the gitlab_main/gitlab_sec schema boundaries. As such, we need to ensure that it is finalised before we are able to proceed with the decomposition.

Additionally, it is one of the migrations which had it's schema changed during the decomposition. As such we need to finalise for both schemas.