composer(deps-dev): Update phpunit/phpunit requirement (#312)

Updates the requirements on [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) to permit the latest version.
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/9.6.15/ChangeLog-9.6.md)
- [Commits](sebastianbergmann/phpunit@9.6.13...9.6.15)

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

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

composer.json

@@ -20,7 +20,7 @@
         "symfony/finder": "^4.4 || ^5.3 || ^6.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^9.6.13",
+        "phpunit/phpunit": "^9.6.15",
         "symfony/phpunit-bridge": "^7.0.0"
     },
     "autoload": {