Resolve "Inconsistency: Repo Title "down caret" cropped on smaller viewports"
## What does this MR do?
Fixes an issue where the dropdown chevron for a project list was not visible on smaller viewports.
This MR also changes the dropdown trigger into a `button` element for better accessibility (it can now be tabbed to like any link).
## Are there points in the code the reviewer needs to double check?
None that I can think of.
## Why was this MR needed?
Better accessibility and support for smaller viewports.
## Screenshots (if relevant)
Dropdown toggle now targetable by pressing tab to select.
Chevron no longer hidden when project title length triggers `overflow: ellipsis`.
## Does this MR meet the acceptance criteria?
- [x] CHANGELOG entry added
- Tests
- [x] All builds are passing
- [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 you do - 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#21506
See merge request !6140
bc13cd7a9b