Update phpunit/phpunit requirement from ^9.6.17 to ^9.6.18

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/9.6.18/ChangeLog-9.6.md)
- [Commits](sebastianbergmann/phpunit@9.6.17...9.6.18)

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

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

composer.json

@@ -34,7 +34,7 @@
         "phpspec/prophecy-phpunit": "^2.2",
         "phpstan/extension-installer": "^1.3.1",
         "phpstan/phpstan": "^1.10",
-        "phpunit/phpunit": "^9.6.17",
+        "phpunit/phpunit": "^9.6.18",
         "squizlabs/php_codesniffer": "^3.9"
     },
     "config": {