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/.github/workflows/backend.yml

@@ -8,8 +8,10 @@ on: [workflow_dispatch, push, pull_request]
 
 jobs:
   run:
-    uses: flarum/.github/.github/workflows/REUSABLE_backend.yml@main
+    uses: flarum/.github/.github/workflows/REUSABLE_backend.yml@as/support-npm-yarn
     with:
       enable_backend_testing: true
 
-      backend_directory: .
+      backend_directory: .
+
+      php_versions: '["7.4", "8.0"]'

extensions/package-manager/composer.json

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