Update phpstan/phpstan requirement from 1.8.11 to 1.9.4 (#59)

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@1.8.11...1.9.4)

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

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

Author: dependabot[bot]

composer.json

@@ -9,7 +9,7 @@
         "ext-reflection": "*"
     },
     "require-dev": {
-        "phpstan/phpstan": "1.8.11",
+        "phpstan/phpstan": "1.9.4",
         "phpstan/extension-installer": "^1.0",
         "phpstan/phpstan-strict-rules": "^1.0",
         "phpunit/phpunit": "^9.5"