Build(deps-dev): Update phpstan/phpstan requirement

Updates the requirements on [phpstan/phpstan](https://github.com/phpstan/phpstan) to permit the latest version.
- [Release notes](https://github.com/phpstan/phpstan/releases)
- [Commits](phpstan/phpstan@0.12.55...0.12.56)

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

composer.json

@@ -31,7 +31,7 @@
     "mimmi20/coding-standard": "~1.0.5",
     "pepakriz/phpstan-exception-rules": "~0.11.2",
     "phpstan/extension-installer": "~1.0.5",
-    "phpstan/phpstan": "~0.12.55",
+    "phpstan/phpstan": "~0.12.56",
     "phpstan/phpstan-deprecation-rules": "~0.12.5",
     "phpstan/phpstan-phpunit": "~0.12.16",
     "phpunit/phpunit": "~7.5.20 || ~8.5.8 || ~9.4.2"