AJAX controller should use CacheableAjaxResponse

Problem/Motivation

QuickTabsController::ajaxContent() uses AjaxResponse, which does not implement CacheableResponseInterface. Cache metadata (tags, contexts) from the
rendered tab content is not bubbled onto the response. Dynamic Page Cache cannot cache the response. Core's ViewAjaxController uses CacheableAjaxResponse and bubbles
cache metadata for the same pattern (see core/modules/views/src/Controller/ViewAjaxController.php lines 148, 222–227).

Proposed resolution

Replace AjaxResponse with CacheableAjaxResponse and bubble cache metadata from the rendered content onto the response, matching what core's Views AJAX controller
does:

$response = new CacheableAjaxResponse();                                                                                                                                                
  $response->addCommand(new HtmlCommand($element_id, $render));                                                                                                                                      
  $response->addCacheableDependency(CacheableMetadata::createFromRenderArray($render));                                                                                                              
  $response->addCacheableDependency($qt);