added php8.1 to test matrix

Author: Daniel Kemper

.github/workflows/main.yaml

@@ -7,7 +7,7 @@ jobs:
         runs-on: ubuntu-latest
         strategy:
             matrix:
-                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']
         name: PHP ${{ matrix.php-versions }}
         steps:
             - name: Checkout