Update phpstan/phpstan requirement from ^1.5 to ^1.6

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/1.7.x/CHANGELOG.md)
- [Commits](phpstan/phpstan@1.5.0...1.6.3)

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

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

composer.json

@@ -64,7 +64,7 @@
         "nyholm/psr7-server": "^1.0",
         "phpspec/prophecy": "^1.15",
         "phpspec/prophecy-phpunit": "^2.0",
-        "phpstan/phpstan": "^1.5",
+        "phpstan/phpstan": "^1.6",
         "phpunit/phpunit": "^9.5",
         "slim/http": "^1.2",
         "slim/psr7": "^1.5",