composer(deps-dev): update phpunit/phpunit requirement

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/12.1.6/ChangeLog-12.1.md)
- [Commits](sebastianbergmann/phpunit@12.1.5...12.1.6)

---
updated-dependencies:
- dependency-name: phpunit/phpunit
  dependency-version: 12.1.6
  dependency-type: direct:development
...

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

Author: dependabot[bot]

composer.json

@@ -36,7 +36,7 @@
     "phpstan/extension-installer": "^1.4.3",
     "phpstan/phpstan": "^2.1.16",
     "phpstan/phpstan-deprecation-rules": "^2.0.3",
-    "phpunit/phpunit": "^12.1.5",
+    "phpunit/phpunit": "^12.1.6",
     "rector/rector": "^2.0.16",
     "rector/type-perfect": "^2.1.0",
     "shipmonk/composer-dependency-analyser": "^1.8.3",