build(deps-dev): update phpunit/phpunit requirement from ^8.5 to ^9.6

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/9.6.19/ChangeLog-9.6.md)
- [Commits](sebastianbergmann/phpunit@8.5.0...9.6.19)

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

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

composer.json

@@ -28,7 +28,7 @@
         "yiisoft/yii2-httpclient": "^2.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^8.5",
+        "phpunit/phpunit": "^9.6",
         "rector/rector": "^1.0"
     },
     "autoload": {