Resolve "Side by side MR discussion should stretch to max width"

What does this MR do?

Fixes a regression whereby the MR discussion note would not expand to full width (Most noticeable on large screens, maybe reproducible when you zoom out in your browser)

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

Make sure this doesn't create any unintended side effects

Why was this MR needed?

Fix regression

Screenshots (if relevant)

Before

After

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
• 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?

Closes #32556 (closed)