[BSv5] Adjust media-breakpoint-down() argument

[chalin]

• Contributes to Upgrade to Bootstrap 5.2 #470
• media-breakpoint-down(): changed argument to be next breakpoint of higher value.

To double check that the breakpoints for say, the <footer>, are the same as before, inspect the <footer> CSS as you change the view width. After this PR lands, the media breakpoint becomes:

@media (max-width: 991.98px)
footer {
    min-height: 200px;
}

Vs. (max-width: 767.98px).