Update phpstan/phpstan requirement from ^2.0 to ^2.1 (#77)

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...2.1.0)

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

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

composer.json

@@ -8,7 +8,7 @@
     },
     "require-dev": {
         "phpstan/extension-installer": "^1.4",
-        "phpstan/phpstan": "^2.0",
+        "phpstan/phpstan": "^2.1",
         "phpunit/phpunit": "^10.5",
         "motley/ecs-config": "^2.1",
         "symfony/var-dumper": "^6.4"