actions: test PHP 8.1

Author: dktapps

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ jobs:
     if: "!contains(github.event.head_commit.message, '[ci skip]')"
     strategy:
       matrix:
-        php: ['7.4', '8.0']
+        php: ['7.4', '8.0', '8.1']
     name: PHP ${{ matrix.php }}
     steps:
     - uses: actions/checkout@v2