Fix markdown tabs alignment on issue/MR create pages

What does this MR do and why?

Fixes a regression in the markdown tabs alignment on issue/MR create pages.

Screenshots or screen recordings

Before	After

How to set up and validate locally

Check description markdown field on New Issue and New Merge request pages.

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 #372220 (closed)