build(deps-dev): update phpstan/phpstan requirement from 1.8.6 to 1.8.9

Updates the requirements on [phpstan/phpstan](https://github.com/phpstan/phpstan) to permit the latest version.
- [Release notes](https://github.com/phpstan/phpstan/releases)
- [Changelog](https://github.com/phpstan/phpstan/blob/1.9.x/CHANGELOG.md)
- [Commits](phpstan/phpstan@1.8.6...1.8.9)

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

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

composer.json

@@ -23,7 +23,7 @@
     "require-dev": {
         "nette/tester": "2.4.3",
         "mockery/mockery": "1.5.1",
-        "phpstan/phpstan": "1.8.6",
+        "phpstan/phpstan": "1.8.9",
         "phpstan/phpstan-strict-rules": "1.4.4",
         "phpstan/phpstan-nette": "1.1.0",
         "phpstan/phpstan-mockery": "1.1.0",