Adjust 404's for LegacyDiffNote discussion rendering

What does this MR do?

Solves #44295 (closed). Further explanation about the issue can be found on https://gitlab.com/gitlab-org/gitlab-ce/issues/44295#note_66706307.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• 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
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #44295 (closed)