Limit repetitive headers in the MR accessibility widget

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA. As a benefit of being a GitLab Community Contributor, you receive complimentary access to GitLab Duo.

What does this MR do and why?

Limit repetitive "New" headers in the MR accessibility widget.

Introduce "Fixed" and "Not fixed" headers for the MR accessibility widget.

Screenshots or screen recordings

Before	After
before.mov	after.mov

How to set up and validate locally

1. Run GitLab.
2. Choose any project with .gitlab-ci.yml and update it according to the docs. You can use this project as an example.
3. Create a merge request which contains an accessibility errors.

Related to #350680 (closed)