Add PHP 8.1 to CI

Author: soatok

.github/workflows/ci.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       matrix:
         operating-system: ['ubuntu-latest']
-        php-versions: ['8.0']
+        php-versions: ['8.0', '8.1']
         phpunit-versions: ['latest']
     steps:
       - name: Checkout