PHPStan: Upgrade to 1.12.6

Author: jakubboucek

composer.json

@@ -14,7 +14,7 @@
         "psr/http-message": "~1.0 || ~2.0"
     },
     "require-dev": {
-        "phpstan/phpstan": "1.10.18",
+        "phpstan/phpstan": "1.12.6",
         "phpunit/phpunit": "^9.0 || ^10.0"
     },
     "suggest": {