Update phpstan/phpstan requirement from ^2.0 to ^2.1

---
updated-dependencies:
- dependency-name: phpstan/phpstan
  dependency-version: 2.1.32
  dependency-type: direct:development
...

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

Author: dependabot[bot]

composer.json

@@ -45,7 +45,7 @@
         "laminas/laminas-diactoros": "^3.1.0",
         "nyholm/psr7": "^1.8.1",
         "php-http/psr7-integration-tests": "^1.3.0",
-        "phpstan/phpstan": "^2.0",
+        "phpstan/phpstan": "^2.1",
         "phpunit/phpunit": "^9.6",
         "doctrine/instantiator": "^1.3.1",
         "squizlabs/php_codesniffer": "^3.9"