chore: Update phpstan.neon.dist to level 6 (#6780)

Co-authored-by: 李铭昕 <715557344@qq.com>

Author: huangdijia

src/Packer/JsonLengthPacker.php

@@ -20,7 +20,7 @@ class JsonLengthPacker implements PackerInterface
 
     protected int $length;
 
-    protected $defaultOptions = [
+    protected array $defaultOptions = [
         'package_length_type' => 'N',
         'package_body_offset' => 4,
     ];