Fix inconsistent Cancel button text

What does this MR do?

Consolidate button text on merge request comment drafts to use "Discard draft" globally. See #47475 (closed)

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

Should we externalize this string via this MR?

Why was this MR needed?

See #47475 (closed) finding from @winh.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes #47475 (closed).