ci: test against Symfony 8.1 (#489)

Author: marmichalski

.github/workflows/ci.yml

@@ -28,9 +28,9 @@ jobs:
         symfony-version:
           - "5.4.*"
           - "6.4.*"
-          - "7.3.*"
           - "7.4.*"
           - "8.0.*"
+          - "8.1.*"
 
     steps:
       - name: "Checkout"