composer(deps-dev): update phpstan/phpstan-phpunit requirement (#178)

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@0.12.20...0.12.21)

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

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

composer.json

@@ -27,7 +27,7 @@
     "phpstan/extension-installer": "^1.1.0",
     "phpstan/phpstan": "^0.12.90",
     "phpstan/phpstan-deprecation-rules": "^0.12.6",
-    "phpstan/phpstan-phpunit": "^0.12.20",
+    "phpstan/phpstan-phpunit": "^0.12.21",
     "phpunit/phpunit": "^9.5.6"
   },
   "config": {