Replace stubbing of `Gitlab.com?` with `:saas` annotation in specs

See https://gitlab.com/gitlab-org/gitlab/-/blob/dd1e70d3676891025534dc4a1e89ca9383178fe7/ee/spec/support/stub_dot_com_check.rb#L4

We have a few of them:

$ git grep :com? | grep _spec.rb | wc -l
274

Shouldn't be too much effort to replace them and/or add a cop?

The following discussion from !89865 (merged) should be addressed:

• @sabrams started a discussion: (+2 comments)

  Not blocking: Just for reference, we have a metadata helper for .com checks that can also help in certain tricky situations.

    it 'schedules background jobs for each batch of container_repository', :saas do