Update phpstan/phpstan requirement from 0.12.48 to 0.12.81

Updates the requirements on [phpstan/phpstan](https://github.com/phpstan/phpstan) to permit the latest version.
- [Release notes](https://github.com/phpstan/phpstan/releases)
- [Commits](phpstan/phpstan@0.12.48...0.12.81)

Signed-off-by: dependabot[bot] <support@github.com>

composer.json

@@ -28,7 +28,7 @@
     },
     "require-dev": {
         "phpunit/phpunit": "^8.0",
-        "phpstan/phpstan": "0.12.48",
+        "phpstan/phpstan": "0.12.81",
         "friendsofphp/php-cs-fixer": "2.18.3",
         "maglnet/composer-require-checker": "^1.1.0 || ^2.0.0",
         "phpro/grumphp-shim": "^0.22.0 || ^1.1"