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