Move hardcoded opacity value in `.btn:disabled` to variable `$btn-disabled-opacity` (#24680)

scss/_buttons.scss

@@ -28,7 +28,7 @@
   // Disabled comes first so active can properly restyle
   &.disabled,
   &:disabled {
-    opacity: .65;
+    opacity: $btn-disabled-opacity;
     @include box-shadow(none);
   }
 

scss/_variables.scss

@@ -357,6 +357,7 @@ $btn-font-weight:             $font-weight-normal !default;
 $btn-box-shadow:              inset 0 1px 0 rgba($white, .15), 0 1px 1px rgba($black, .075) !default;
 $btn-focus-width:             $input-btn-focus-width !default;
 $btn-focus-box-shadow:        $input-btn-focus-box-shadow !default;
+$btn-disabled-opacity:        .65 !default;
 $btn-active-box-shadow:       inset 0 3px 5px rgba($black, .125) !default;
 
 $btn-link-disabled-color:     $gray-600 !default;