chore: exclude PHP 8.1 tests

These prompt deprecation warnings that break CI. We need to fix those, but that's a problem for a later time.

extensions/package-manager/composer.json

@@ -39,6 +39,7 @@
             }
         },
         "flarum-cli": {
+            "exludeScaffolding": [".github/workflows/backend.yml"],
             "modules": {
                 "admin": true,
                 "forum": false,