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/_pagination.scss

@@ -34,7 +34,7 @@
   &.active .page-link {
     z-index: 3;
     color: $pagination-active-color;
-    background-color: $pagination-active-bg;
+    @include gradient-bg($pagination-active-bg);
     border-color: $pagination-active-border-color;
   }