Fix the fork project functionality for projects with hashed storage

What does this MR do?

Switches from the gitlab-shell fork-project command to fork-repository. This allows projects using hashed storage to be forked.

Are there points in the code the reviewer needs to double check?

Previously, gitlab-shell would refuse to create missing parent directories. Now it creates them. They're expected to be missing for hashed storage, I think.

Why was this MR needed?

Forking projects backed by hashed storage is currently broken. This is No Good.

Screenshots (if relevant)

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
• Internationalization required/considered

What are the relevant issue numbers?

Closes #40711 (closed)