Move CreateRepository to OPT_OUT for Gitaly

What does this MR do?

2 things;

1. Move Gitlab::Shell#create_repository to OPT_OUT
2. Rename Gitlab::Shell#add_repository to #create_repository

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated
• [-] API support added
• [-] Tests added for this feature/bug
• Review
  • [-] Has been reviewed by UX
  • [-] Has been reviewed by Frontend
  • Has been reviewed by Backend
  • [-] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

gitaly#593 (closed)