Migrate merge request list and edit pages to container queries

What does this MR do and why?

This MR migrates existing CSS on the merge request list and merge request edit/create pages to use container queries instead of media queries.

It also cleans up some outdated code.

It does not migrate Bootstrap classes, which still rely on media queries.

No visual change expected.

How to set up and validate locally

1. Open merge request list
2. Observe no visual regressions
3. Click 'New merge request'
4. Observe no visual regressions
5. Edit any merge request
6. Observe no visual regressions
7. Enable tailwind_container_queries feature flag
8. Repeat steps 1-6