Add PHP 8.1 to CI

Author: hbgl

.github/workflows/ci.yml

@@ -6,7 +6,7 @@ jobs:
     strategy:
       matrix:
         operating-system: [ubuntu-latest]
-        php-versions: ['7.1', '7.2', '7.3', '7.4', '8.0']
+        php-versions: ['7.1', '7.2', '7.3', '7.4', '8.0', '8.1']
     runs-on: ${{ matrix.operating-system }}
     steps:
       - name: Checkout