Remain ChatApp .NET Framework (.NET Framework 462), but can be work with .NET Standard 2.1, too.
ChatApp.Shared to reference MessagePack.Annotations, instead of MessagePack.
Use NuGetForUnity to install Dependency dlls, instead of direct dll install.
Packages updates.
Install NuGetForUnity via git UPM.
Use Latest YetAnotherHttpHandlerr 1.2.0
[!NOTE]
MessagePack installation notice
If you use NuGetForUnity to install MessagePack, you can't reference MessagePack.Unity.UnityResolver.Instance because NuGet's MessagePack doesn't include this.
To resolve this issue, use MessagePack's .unitypackage.
tl;dr;
.NET Framework(.NET Framework 462), but can be work with.NET Standard 2.1, too.ChatApp
Works fine with Unity 2021.3.0f1