Update phpstan/phpstan requirement from ^1.0 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](https://github.com/phpstan/phpstan/compare/1.0.0...2.1.0)

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

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

composer.json

@@ -18,7 +18,7 @@
   "require-dev": {
     "phpunit/phpunit": "^6.0|^9.5",
     "friendsofphp/php-cs-fixer": "^3.0",
-    "phpstan/phpstan": "^1.0"
+    "phpstan/phpstan": "^2.1"
   },
   "autoload": {
     "psr-4": {