composer(deps-dev): update phpunit/phpunit requirement

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@9.5.28...9.6.0)

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

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

composer.json

@@ -23,7 +23,7 @@
     },
     "require-dev": {
         "orchestra/testbench": "^6.25.1",
-        "phpunit/phpunit": "^9.5.28"
+        "phpunit/phpunit": "^9.6.0"
     },
     "minimum-stability": "dev",
     "prefer-stable": true,