composer(deps-dev): update phpunit/phpunit requirement (#283)

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/master/ChangeLog-9.5.md)
- [Commits](sebastianbergmann/phpunit@9.5.17...9.5.18)

---
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>

composer.json

@@ -36,7 +36,7 @@
     "phpstan/phpstan": "^1.4.8",
     "phpstan/phpstan-deprecation-rules": "^1.0.0",
     "phpstan/phpstan-phpunit": "^1.0.0",
-    "phpunit/phpunit": "^9.5.17"
+    "phpunit/phpunit": "^9.5.18"
   },
   "minimum-stability": "stable",
   "prefer-stable": true,