State PHP compatibility

The package most probably is compatible with more versions, but we use
8.1.x right now.
Also the package didn't communicate any compatibility until now.

composer.json

@@ -9,6 +9,7 @@
         }
     ],
     "require": {
+        "php": "~8.1.0",
         "ericmartel/codeception-email": "^1.0",
         "guzzlehttp/guzzle": "^6.1 || ^7.0"
     },
@@ -18,6 +19,7 @@
         }
     },
     "require-dev": {
-        "friendsofphp/php-cs-fixer": "^3.22"
+        "friendsofphp/php-cs-fixer": "^3.22",
+        "phpstan/phpstan": "^1.10"
     }
 }