Add php 8.1 to build matrix

Author: jaapio

.github/workflows/push.yml

@@ -88,7 +88,7 @@ jobs:
           - ubuntu-latest
           - windows-latest
           - macOS-latest
-        php-versions: ['7.2', '7.3', '7.4', '8.0']
+        php-versions: ['7.2', '7.3', '7.4', '8.0', '8.1']
     name: Unit tests for PHP version ${{ matrix.php-versions }} on ${{ matrix.operating-system }}
     needs:
       - setup