Add PHP 8.1 in CI

Author: tgalopin

.github/workflows/unit-tests.yaml

@@ -10,7 +10,7 @@ jobs:
         runs-on: ubuntu-latest
         strategy:
             matrix:
-                php-versions: ['7.2', '7.3', '7.4', '8.0']
+                php-versions: ['7.2', '7.3', '7.4', '8.0', '8.1']
         steps:
             - name: Checkout
               uses: actions/checkout@v2