Move Gitlab::Git::Repository#add_branch to mandatory

What does this MR do?

Move Gitlab::Git::Repository#add_branch to mandatory

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-and-qa manual pipeline job)