v4: Navbar improvements

Switch to separate vars for alert vertical/horizontal padding

Add vertical-align utilities

- Adds six new utils for vertical-align property
- Documents the additional classes in the utils docs

Follow up to 6e8ce0c7. Uses the breakpoint-next mixin to snag the next breakpoint in the media query for proper styling.

- Moves .navbar-brand margin and float to media queries
- Replaces the static media queries with an each loop to generate all the needed classes

Revert "Drop the $small-font-size variable for the newer $font-size-sm one"

Change SauceLabs iPhone test to use Appium.

Update scrollspy navbar example.

Fixed #20897 by changing btn-default to btn-secondary in docs for V4

v4: Fix outdated utilites classes

Convert Collapse accordion from Panels to Cards