Update phpunit/phpunit requirement from ^9.6.15 to ^9.6.16

Updates the requirements on [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) to permit the latest version.
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/9.6.16/ChangeLog-9.6.md)
- [Commits](sebastianbergmann/phpunit@9.6.15...9.6.16)

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

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

composer.json

@@ -53,7 +53,7 @@
         "phpspec/prophecy-phpunit": "^2.1",
         "phpstan/extension-installer": "^1.3.1",
         "phpstan/phpstan": "^1.10",
-        "phpunit/phpunit": "^9.6.15",
+        "phpunit/phpunit": "^9.6.16",
         "squizlabs/php_codesniffer": "^3.8"
     },
     "config": {