Fix project transfer with hashed-storage attachments

What does this MR do?

Skips the "move uploads" step for hashed projects being transferred between namespaces

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

Why was this MR needed?

Without this, transferring a hashed project will either break its attachments or fail the transfer, I've not checked which.

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?

/cc @brodock @stanhu

Closes gitlab-ee#4056