Align milestone column header with count number

## What does this MR do?
- Aligns the milestone column header with the count number
- Prevents the count from wrapping below the column header

## Why was this MR needed?
- Count number was being pushed below the column header text if the text was too long

## Screenshots (if relevant)
- see issue sceenshots for before #25810 
- fixed version
![milestones](/uploads/ca12c290abe814b457ad3b2fb86f7696/milestones.png)


## Does this MR meet the acceptance criteria?

- [ ] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?


Closes #25810

See merge request !8177