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/_utilities.scss

@@ -416,6 +416,11 @@ $utilities: map-merge(
         )
       )
     ),
+    "gradient": (
+      property: background-image,
+      class: bg,
+      values: (gradient: var(--bs-gradient))
+    ),
     "white-space": (
       property: white-space,
       class: text,