Test upgrading phpstan

Author: smnandre

.github/workflows/ci.yml

@@ -54,7 +54,7 @@ jobs:
         with:
           php-version: 8.3
           coverage: none
-          tools: phpstan:1.10, cs2pr
+          tools: phpstan:2.1, cs2pr
 
       - name: Checkout code
         uses: actions/checkout@v4