build(deps-dev): update phpstan/phpstan requirement from ^1.12 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@1.12.0...2.1.1)

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

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

composer.json

@@ -39,7 +39,7 @@
     "require-dev": {
         "phpunit/phpunit": "^10.5",
         "symfony/config": "^6.0 || ^7.0",
-        "phpstan/phpstan": "^1.12",
+        "phpstan/phpstan": "^2.1",
         "vimeo/psalm": "^5.26",
         "psalm/plugin-phpunit": "^0.19.0",
         "symfony/phpunit-bridge": "^6.0 || ^7.0"