Update tests to run under PHP 8.1

Author: dd32

.github/workflows/unit-test.yml

@@ -12,7 +12,7 @@ jobs:
           fail-fast: false
           matrix:
               php:
-                - '7.4'
+                - '8.1'
 
       env:
         WP_ENV_PHP_VERSION: ${{ matrix.php }}