Add Symfony 8.1 to CI

Author: HypeMC

.github/workflows/tests.yaml

@@ -14,7 +14,7 @@ jobs:
             fail-fast: false
             matrix:
                 php: [ '8.1', '8.2', '8.3', '8.4', '8.5' ]
-                symfony: [ '6.4', '7.4', '8.0' ]
+                symfony: [ '6.4', '7.4', '8.0', '8.1' ]
                 dependencies: [ 'highest', 'lowest' ]
                 exclude:
                     -   php: '8.1'