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...2.1.17)

---
updated-dependencies:
- dependency-name: phpstan/phpstan
  dependency-version: 2.1.17
  dependency-type: direct:development
...

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

Author: dependabot[bot]

composer.json

@@ -64,7 +64,7 @@
         "nyholm/psr7-server": "^1.1",
         "phpspec/prophecy": "^1.19",
         "phpspec/prophecy-phpunit": "^2.1",
-        "phpstan/phpstan": "^1.12",
+        "phpstan/phpstan": "^2.1",
         "phpunit/phpunit": "^9.6",
         "slim/http": "^1.3",
         "slim/psr7": "^1.6",