Issue #3480092: Added fullscreen to allow policy + removed allowfullscreen legacy attribute.

Related to #3480092

src/Plugin/Field/FieldFormatter/IframeDefaultFormatter.php

@@ -231,10 +231,10 @@ class IframeDefaultFormatter extends FormatterBase {
     $allow[] = 'microphone';
     $allow[] = 'payment';
     $allow[] = 'picture-in-picture';
-    $options['allow'] = implode(';', $allow);
     if (!empty($item->allowfullscreen)) {
-      $options['allowfullscreen'] = 'allowfullscreen';
+      $allow[] = 'fullscreen';
     }
+    $options['allow'] = implode(';', $allow);
 
     if ($this->moduleHandler->moduleExists('token')) {
       // Token Support for field "url" and "title".