Enable PHP 8.1 CI

Author: williamdes

.github/workflows/tests.yml

@@ -13,7 +13,7 @@ jobs:
         runs-on: ubuntu-latest
         strategy:
             matrix:
-                php-version: ["7.1", "7.2", "7.3", "7.4", "8.0"]
+                php-version: ["7.1", "7.2", "7.3", "7.4", "8.0", "8.1"]
         steps:
             - uses: actions/checkout@v3
             - name: Install xmllint