[TOOLS] Raise PHPStan level to 5 and fix associated error, fixing some bugs in the process

plugins/Pinboard/Controller/APIv1.php

@@ -73,7 +73,7 @@ class APIv1 extends Controller
             // dd($tag_names, $keys, $result, $xml, (string) $xml);
             // $xml->addChild();
             // dd($xml);
-            return new Response(content: $xml, status: $status);
+            return new Response(content: (string) $xml, status: $status);
         } else {
             throw new InvalidRequestException;
         }