Merge pull request #584 from FriendsOfSymfony/test-php81

test with php 8.1

Author: dbu

.github/workflows/php.yml

@@ -20,6 +20,7 @@ jobs:
           - php-version: '7.3'
           - php-version: '7.4'
           - php-version: '8.0'
+          - php-version: '8.1'
           - php-version: '7.4'
             symfony-version: '4.*'
           - php-version: '7.4'