Zend\Diactoros\Response\JsonResponse not in core 9.3.6.

Problem/Motivation

The ContentReadabilityAjaxController.php returns a Zend\Diactoros\Response\JsonResponse, which is no longer available in core 9.3.6.

Steps to reproduce

Update to core 9.3.6.

Errors will be logged about a missing class when attempting to use Content Readability in a body field.

Proposed resolution

Replace with Symfony\Component\HttpFoundation\JsonResponse.

Remaining tasks

Create merge request with changes.