Fixes #26465

Fixes #27595

* Tweak the accessibility/reduced motion text

include mention of carousel slides, remove the (now inaccurate, as Firefox 63 includes it too) mention that support is limited to Safari/macOS

xref https://github.com/twbs/bootstrap/issues/27525

* Add new callout for reduced motion

* Add variable to control prefers-reduced-motion media query support

* Add callout about prefers-reduced-motion to all components currently using animation which are affected

This change follows exactly what was done for the `$grays`, `$colors`, and `$theme-colors` lists in #23260. This allows for easy addition of new breakpoints or grid containers also.

Define new variable for card header color

In case we set `$body-bg` to a dark color, we have to keep table contrast relevant.

* #26967: Allow multiple ways of padding declaration for modal header
* #26976: split vertical and horizontal modal padding

Fixes #26886

* style: `_variables.scss` comments consistency

* style: `_variables.scss` spacing consistency