[Controller] Implement "Revision" in CR status using `metadata.generation`

Summary

Similar to the implementation in the previous operator, we use the GitLab CR's metadata.generation (which increments upon changes to the CR spec) as the status.revision field (more information below).

This enables users to clearly see which revision of GitLab is running, and we can use that revision in various ways in the controller - such as log entries, resource names, etc.

Acceptance criteria

• The GitLab CR's Status object is updated with the revision from metadata.generation
• (Optional) This Revision is used in the Migrations jobs (the solution we have today with the first 3 characters from the template hash may be sufficient and not worth changing - up for discussion)