Update phpunit/phpunit requirement from ^9.6.21 to ^11.4.1

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/11.4.1/ChangeLog-11.4.md)
- [Commits](sebastianbergmann/phpunit@9.6.21...11.4.1)

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

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

composer.json

@@ -20,7 +20,7 @@
         "psr-4": { "League\\ISO3166\\": "src" }
     },
     "require-dev": {
-        "phpunit/phpunit": "^9.6.21",
+        "phpunit/phpunit": "^11.4.1",
         "phpstan/phpstan": "^1.12.6",
         "phpstan/phpstan-deprecation-rules": "^1.2.1",
         "phpstan/phpstan-strict-rules": "^1.6.1"