Add placeholder in the color input inside the create new label popup

## What does this MR do?
Adds the missing placeholder  *"Assign custom color #FF0000"*  in the label creation popup ( example text for custom hex color).

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

## Why was this MR needed?
When creating a new label, the example text for custom hex color is missing

## Screenshots (if relevant)
Before
![before](/uploads/a3157414d59cea609118a7d711f9b9ee/before.png)

After
![Selection_051](/uploads/ca3e2089b40069e4cb6b7aa295b28cfc/Selection_051.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)
- [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?
#24512 

See merge request !7492