Update phpunit/phpunit requirement from ^9.6 to ^11.3

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/11.3.6/ChangeLog-11.3.md)
- [Commits](sebastianbergmann/phpunit@9.6.0...11.3.6)

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

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

composer.json

@@ -28,7 +28,7 @@
         "squizlabs/php_codesniffer": "3.*",
         "phpmd/phpmd": "^2.11",
         "oxid-esales/oxideshop-ce": "^7.1",
-        "phpunit/phpunit": "^9.6",
+        "phpunit/phpunit": "^11.3",
         "mikey179/vfsstream": "~1.6.8",
         "codeception/codeception": "*",
         "codeception/module-asserts": "*",