API Stop using deprecated API

Author: emteknetnz

src/Extensions/GridFieldDetailFormItemRequestExtension.php

@@ -64,7 +64,7 @@ private function getHTMLFragment(GridFieldAddNewMultiClass $component)
                 'discard-confirmation',
                 'action--new__multi-class',
             )),
-            'data-classes' => Convert::array2json($classes),
+            'data-classes' => json_encode($classes),
         ]);
     }