Fix production admin fixture to use the new `Users::CreateService`

What does this MR do?

This change the production admin fixture to use the new Users::CreateService.

Are there points in the code the reviewer needs to double check?

The trick used here is to pass a transient non-persisted admin user to the service as the "current user" since only admins can create other admins! :)

Does this MR meet the acceptance criteria?

• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #30248 (closed)

/cc @twk3 @geoandri

approved this merge request

enabled an automatic merge when the pipeline for 53ef1de4 succeeds

Makes sense, nice fix!

canceled the automatic merge

Both failures are transient and unrelated (one is https://gitlab.com/gitlab-org/gitlab-ce/issues/30236); merging!

merged

mentioned in commit c7403521