Be explicit on merge request discussion variables

What does this MR do?

To avoid conditionals and to messing with request.format and accept headers to know in which format we're going to response I've decided to be explicit in when we need the discussion variables

Why was this MR needed?

Solve a bug https://sentry.gitlap.com/gitlab/staginggitlabcom/issues/8492/

What are the relevant issue numbers?

Closes #19702 (closed)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together