feat: Update package.json to 1.21.4

Commit by [GitHub Actions](https://github.com/Cysharp/MemoryPack/actions/runs/13177712265)

Author: github-actions[bot]

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

@@ -2,7 +2,7 @@
     "name": "com.cysharp.memorypack",
     "displayName": "MemoryPack",
     "author": { "name": "Cysharp, Inc.", "url": "https://cysharp.co.jp/en/" },
-    "version": "1.21.3",
+    "version": "1.21.4",
     "unity": "2022.3",
     "unityRelease": "12f1",
     "description": "Zero encoding extreme performance binary serializer for C#.",