- Use a semitransparent gradient from light to dark which works on any background-color
- Store the gradient as a custom property (--bs-gradient)
- Remove `.bg-gradient-*` variants in favour of `.bg-gradient` which works even when `$enable-gradients` are enabled
- Add gradients to navbar, active page links and badges when gradients are enabled
Media queries do not support CSS custom properties (CSS variables), so we've had some useless variables lying around since shipping v4. This finally removes them and the associated documentation content that explained the situation.
Fixes#29012.
