Update phpstan/phpstan requirement from ^2.0 to ^2.1

Updates the requirements on [phpstan/phpstan](https://github.com/phpstan/phpstan) to permit the latest version.
- [Release notes](https://github.com/phpstan/phpstan/releases)
- [Changelog](https://github.com/phpstan/phpstan/blob/2.1.x/CHANGELOG.md)
- [Commits](phpstan/phpstan@2.0.0...2.1.1)

---
updated-dependencies:
- dependency-name: phpstan/phpstan
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

composer.json

@@ -17,7 +17,7 @@
         "composer-runtime-api": "^2",
         "composer/semver": "^3.4",
         "nikolaposa/version": "^4.1",
-        "phpstan/phpstan": "^2.0"
+        "phpstan/phpstan": "^2.1"
     },
     "require-dev": {
         "friendsofphp/php-cs-fixer": "^3.65",