What does this MR do and why?

Updates the merge request header by moving the branch information from the widget into the header.

Screenshots or screen recordings

How to set up and validate locally

Enable updated_mr_header

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 #356703 (closed)