Fix #26372: disabled btn hover issue (#27407)

When gradients are enabled there is still a hover state on disabled
buttons since the hover rules apply to background-image and disabled
rules apply to background-color. This applies the logic already present
in dropdowns to buttons. This fix was originally proposed by @ysds.

scss/mixins/_buttons.scss

@@ -31,6 +31,10 @@
     color: color-yiq($background);
     background-color: $background;
     border-color: $border;
+    // Remove CSS gradients if they're enabled
+    @if $enable-gradients {
+      background-image: none;
+    }
   }
 
   &:not(:disabled):not(.disabled):active,