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/_forms.scss

@@ -97,7 +97,7 @@
       border-color: $color;
 
       &:checked {
-        @include gradient-bg(lighten($color, 10%), escape-svg($form-check-input-checked-bg-image));
+        background-color: $color;
       }
 
       &:focus {