new system note design for commit discussion

## What does this MR do?
This MR implement new system notes design for discussion on a commit.

## Are there points in the code the reviewer needs to double check?
NR
## Why was this MR needed?
It improves UX and coherent style in UI
## Screenshots (if relevant)
**before**
![commit-discussion-before](/uploads/b68ab7cce583a18ac066691448f955e7/commit-discussion-before.png)

**After**
![commit-discussion-after](/uploads/e9c0cb48c29fdd9b6de6f23a3c588d03/commit-discussion-after.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
  - [x] 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)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?


Closes #24894

See merge request !7721