Fix incorrect naming in code related to cross-references

The following discussion from !72794 (merged) should be addressed:

• @alexkalderimis started a discussion: (+1 comment)

  I notice that the signature for this method is defined as:

      def create_cross_reference_note(mentioned, noteable, author)
        # implement inside child
      end

  the mismatch between noteable and mentionable is a bit confusing. Is it intentional?

  Looking through the code path again, it seems we always have one Mentionable (the mentioner) mentioning another Mentionable (the mentioned), and noteables aren't really directly involved (they just tend to be mentionables too 😅):

  • The entry point is Mentionable#create_cross_references!, which gathers the referenced mentionables at https://gitlab.com/gitlab-org/gitlab/blob/106972a833c7dc5559f09d4632dc6c1f79740572/app/models/concerns/mentionable.rb#L182 (only issues/MRs/commits at the moment)
  • The switch from ref (the mentionable) to noteable happens at SystemNoteService.cross_reference