Use default:selection handler

Closes #2987189

src/Form/BanUserForm.php

@@ -44,7 +44,10 @@ class BanUserForm extends FormBase {
       '#selection_handler' => 'default:user',
       '#selection_settings' => [
         'include_anonymous' => FALSE,
-        'private_message:not_blocked_user' => TRUE,
+        // @see \private_message_entity_query_user_alter()
+        'private_message' => [
+          'active_users_selection' => TRUE,
+        ],
       ],
     ];