explicitly pass NULL to Symfony Response component `setExpires()`.

Closes #3479244

src/Controller/StatuspageController.php

@@ -77,7 +77,7 @@ class StatuspageController {
 
     // Disable browser cache:
     $response->setMaxAge(0);
-    $response->setExpires();
+    $response->setExpires(NULL);
 
     return $response;
   }