Improve gradients

- 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

scss/_mixins.scss

@@ -30,7 +30,6 @@
 @import "mixins/table-variants";
 
 // Skins
-@import "mixins/background-variant";
 @import "mixins/border-radius";
 @import "mixins/box-shadow";
 @import "mixins/gradients";