Pin gemnasium db
Description
Pin gemnasium-db using GEMNASIUM_DB_REF_NAME so that developers won't have to constantly update expectations
GEMNASIUM_DB_REF_NAME is branch name in gemnasium db. Format is YYYY-MM-DD
Related issue
gitlab-org/gitlab#39416 (closed)
Testing
These variables tested in
MR above uses custom image to print variables to observe values of the variables
https://gitlab.com/gitlab-org/security-products/tests/ruby-bundler/-/jobs/403570588 (check line 198)