button.js: Set disabled property in addition to disabled attribute to preserve behavior under jQuery 3 (#20278)

This code ought to have used .prop instead of .attr in the first place,
but we can't get rid of the attr manipulation now due to backward compatibility constraints.

Refs https://github.com/jquery/jquery-migrate/blob/3.0.0/warnings.md#jqmigrate-jqueryfnremoveattr-no-longer-sets-boolean-properties
Refs #16834

[skip validator]

js/button.js

@@ -41,10 +41,10 @@
 
       if (state == 'loadingText') {
         this.isLoading = true
-        $el.addClass(d).attr(d, d)
+        $el.addClass(d).attr(d, d).prop(d, true)
       } else if (this.isLoading) {
         this.isLoading = false
-        $el.removeClass(d).removeAttr(d)
+        $el.removeClass(d).removeAttr(d).prop(d, false)
       }
     }, this), 0)
   }