Resolve "Merge requests tabs should extend the full width when they become fixed"

What does this MR do?

This used to allow the sticky tabs to be full width.

We now have 2 MR tabs in the DOM and toggle the sticky and unsticky one.

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

Why was this MR needed?

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
  • 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 #34153 (closed)