Use the `border-color` property instead of the CSS variable in border color utilities

Fixes #36087 (closed).

Also adds a note to our docs about how these utilities are slightly different than .text-* and .bg-* utilities in that they do not come with an additional --bs-border-opacity CSS variable as that variable is already present on .border-* classes, and not applicable (yet) on other components where these utilities may be used presently.

/cc @julien-deramond