Update a call to a deprecated method

Reported by a GitHub action:
https://github.com/wikimedia/jquery.uls/pull/478/files#file-src-jquery-uls-core-js-L331

src/jquery.uls.core.js

@@ -328,7 +328,7 @@
 
 			if ( e.keyCode === 27 ) { // escape
 				this.cancel();
-				this.$element.focus();
+				this.$element.trigger( 'focus' );
 				e.preventDefault();
 				e.stopPropagation();
 			}