Resolve #3497021 "Deprecate getformname"

Closes #3497021

core/lib/Drupal/Core/Entity/ContentEntityConfirmFormBase.php

@@ -43,6 +43,11 @@ public function getCancelText() {
    * {@inheritdoc}
    */
   public function getFormName() {
+    @trigger_error(
+      'ConfirmFormBase::getFormName() is deprecated in drupal:11.2.0 and will be removed in drupal:12.0.0. 
+      Implementations should remove this method. See https://www.drupal.org/node/3505037',
+      E_USER_DEPRECATED
+    );
     return 'confirm';
   }