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/_variables.scss

@@ -222,6 +222,11 @@ $enable-negative-margins:     false !default;
 $enable-deprecation-messages: true !default;
 $enable-important-utilities:  true !default;
 
+// Gradient
+//
+// The gradient which is added to components if `$enable-gradients` is `true`
+// This gradient is also added to elements with `.bg-gradient`
+$gradient: linear-gradient(180deg, rgba($white, .15), rgba($white, 0)) !default;
 
 // Spacing
 //