Remove extra space in create MR diff

What does this MR do and why?

Removes the gap between the top nav and fixed file header

Screenshots or screen recordings

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

• Navigate to Create merge request form (with changes)
• Click on Changes tab at bottom
• Scroll down so file header becomes sticky

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #370263 (closed)