Update phpunit/phpunit requirement from ^9.5 to ^9.6

Updates the requirements on [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) to permit the latest version.
- [Release notes](https://github.com/sebastianbergmann/phpunit/releases)
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/main/ChangeLog-9.6.md)
- [Commits](sebastianbergmann/phpunit@9.5.0...9.6.4)

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

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

composer.json

@@ -46,7 +46,7 @@
         "nyholm/psr7": "^1.5",
         "php-http/psr7-integration-tests": "dev-master",
         "phpstan/phpstan": "^1.9",
-        "phpunit/phpunit": "^9.5",
+        "phpunit/phpunit": "^9.6",
         "squizlabs/php_codesniffer": "^3.7"
     },
     "autoload": {