Updates the font weight of button styles because of the change to system fonts

## What does this MR do?

Updates the font weight of button styles because of the change to system fonts

## Are there points in the code the reviewer needs to double check?
-

## Why was this MR needed?

Because of the change to system fonts the font weight for buttons needed to be toned down a bit.

Seems only buttons styles were affected.

## Screenshots (if relevant)

left = result

![image](/uploads/c75868dca76561cafd559fb721be0601/image.png)

## Does this MR meet the acceptance criteria?

- [x] [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
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?


cc: @cperessini

See merge request !8012