Update phpunit/phpunit requirement from ^9 to ^11

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/11.2.6/ChangeLog-11.2.md)
- [Commits](sebastianbergmann/phpunit@9.0.0...11.2.6)

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

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

composer.json

@@ -63,7 +63,7 @@
   },
   "require-dev": {
     "prooph/pdo-event-store": "^1.12",
-    "phpunit/phpunit": "^9",
+    "phpunit/phpunit": "^11",
     "symfony/yaml" : "^5.4 || ^6.4 || ^7.0",
     "bookdown/bookdown": "^1.1",
     "prooph/bookdown-template": "^0.3",