[PHP]: Update phpstan/phpstan-phpunit requirement from 1.3.16 to 1.4.0

Updates the requirements on [phpstan/phpstan-phpunit](https://github.com/phpstan/phpstan-phpunit) to permit the latest version.
- [Release notes](https://github.com/phpstan/phpstan-phpunit/releases)
- [Commits](phpstan/phpstan-phpunit@1.3.16...1.4.0)

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

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

composer.json

@@ -30,7 +30,7 @@
         "ext-ast": "*",
         "bamarni/composer-bin-plugin": "^1.4",
         "phpstan/phpstan": "1.11.0",
-        "phpstan/phpstan-phpunit": "1.3.16",
+        "phpstan/phpstan-phpunit": "1.4.0",
         "rector/rector": "1.0.5",
         "roave/security-advisories": "dev-master",
         "serendipity_hq/rector-config": "^1.0",