build(deps-dev): bump phpunit/phpunit from 9.6.7 to 9.6.8

Bumps [phpunit/phpunit](https://github.com/sebastianbergmann/phpunit) from 9.6.7 to 9.6.8.
- [Changelog](https://github.com/sebastianbergmann/phpunit/blob/9.6.8/ChangeLog-9.6.md)
- [Commits](https://github.com/sebastianbergmann/phpunit/compare/9.6.7...9.6.8)

---
updated-dependencies:
- dependency-name: phpunit/phpunit
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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

composer.json

@@ -59,7 +59,7 @@
     },
     "require-dev": {
         "symfony/stopwatch": "5.4.21",
-        "phpunit/phpunit": "9.6.7",
+        "phpunit/phpunit": "9.6.8",
         "roave/security-advisories": "dev-master"
     }
 }