feat: Update package.json to 1.8.9

Author: github-actions[bot]

src/MemoryPack.Unity/Assets/Plugins/MemoryPack/package.json

@@ -2,7 +2,7 @@
     "name": "com.cysharp.memorypack",
     "displayName": "MemoryPack",
     "author": { "name": "Cysharp, Inc.", "url": "https://cysharp.co.jp/en/" },
-    "version": "1.8.8",
+    "version": "1.8.9",
     "unity": "2021.3",
     "description": "Zero encoding extreme performance binary serializer for C#.",
     "keywords": [ "Serialization" ],