Carousel: use buttons, not links, for prev/next controls (#32627)

Carousel: use buttons, not links, for prev/next controls (#32627)

* Carousel: use buttons, not links, for prev/next

- expand the styles to neutralise border/background
- change docs page
- add extra unit test to check that links or buttons work as controls
- modify visual test to use buttons as well
- use buttons instead of links for prev/next
- remove `role="button"` from links that are actually links

* Clarify that controls can be button or link

* Update site/content/docs/5.0/components/carousel.md

Co-authored-by: Mark Otto <markd.otto@gmail.com>
* Explicitly set padding to 0 to prevent dipping/moving on active in Firefox

Co-authored-by: XhmikosR <xhmikosr@gmail.com>