Resolve "UI: Move "default branch" setting under "repository" settings"

What does this MR do?

This moves the "Default Branch" settings under "Repository".

Notes:

• EE Port (gitlab-ee!6992)

TODO

• Fix existing general settings rspec
• Write rspec for repository settings with project with branch
• Write rspec for repository settings with empty project

What are the relevant issue numbers?

Closes #36048 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides