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

Updates the requirements on [phpstan/phpstan](https://github.com/phpstan/phpstan) to permit the latest version.

Updates `phpstan/phpstan` to 1.12.6
- [Release notes](https://github.com/phpstan/phpstan/releases)
- [Changelog](https://github.com/phpstan/phpstan/blob/2.0.x/CHANGELOG.md)
- [Commits](phpstan/phpstan@1.12.5...1.12.6)

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

Signed-off-by: dependabot[bot] <[email protected]>

composer.json

@@ -36,7 +36,7 @@
     "mimmi20/coding-standard": "^5.2.44",
     "nikic/php-parser": "^4.19.1 || ^5.0.2",
     "phpstan/extension-installer": "^1.4.3",
-    "phpstan/phpstan": "^1.12.5",
+    "phpstan/phpstan": "^1.12.6",
     "phpstan/phpstan-deprecation-rules": "^1.2.1",
     "phpstan/phpstan-phpunit": "^1.4.0",
     "phpunit/phpunit": "^10.5.25",