Missing explicit dependency on parser gem in Gemfile

Even though gitlab directly require parser/ruby27 in https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/background_migration/fix_vulnerability_occurrences_with_hashes_as_raw_metadata.rb#L3 it is not listed in Gemfile.

It is indirectly pulled in only in Gemfile.lock https://gitlab.com/gitlab-org/gitlab/-/blob/master/Gemfile.lock#L653

Please add parser gem directly to Gemfile.