Resolve "Forking with namespace doesn't work (API)"

What does this MR do?

Fixes an issue where nested namespace names could not be used when forking projects via the API. I replaced find_by_path_or_name (which does not work with nested paths) with find_by_full_path (which does work with nested paths).

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

#40798 (closed)