Update phpunit/phpunit requirement from ^9.5 to ^9.6 (#94)

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...9.6.3)

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

composer.json

@@ -18,7 +18,7 @@
     "require-dev": {
         "phpstan/extension-installer": "^1.2",
         "phpstan/phpstan": "^1.9",
-        "phpunit/phpunit": "^9.5",
+        "phpunit/phpunit": "^9.6",
         "symfony/var-dumper": "^5.3",
         "thecodingmachine/phpstan-safe-rule": "^1.2",
         "tomasvotruba/cognitive-complexity": "^0.0.5",