Prevent user from changing username with container registry tags

What does this MR do?

Adds validation to prevent a user from changing their username when they have a project with container registry tags, because Namespace#move_dir doesn't work properly in this case.

The check for container registry tags is looking in the new path rather than the old during a user rename, because it happens User#after_update => Namespace#after_update. The container registry stuff doesn't get moved to the new namespace.

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

• I first attempted to fix ContainerRepository#path, but at that point in the transaction, the old path is already lost. So it was simpler to implement the user validation instead
• Group path changes already fail correctly with registry tags (see screenshot)

Screenshots (if relevant)

Username change with tags

Group already works

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• 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
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Fixes #35451 (closed)