Set php-version to v8.1

Author: alexdebril

.github/workflows/ci.yml

@@ -26,6 +26,7 @@ jobs:
       - name: PHPUnit Tests
         uses: php-actions/phpunit@v2
         with:
+          php-version: 8.1
           php_extensions: xdebug
           bootstrap: tests/bootstrap.php
           configuration: phpunit.xml.dist