Dotnet6 support

[hylinux]

I found the library is not support .Net 6 when I used it working on my WeChat project. so, I did some improve:

1. support .Net 6 only.
2. change async code , remove ConfigureAwait(false) since it have some problem in Asp.net Core 6
3. change sync code in IO. sync IO is not support after Asp.net core 3.
4. copy old Demo and create a new one which is base on Asp.net Core 6.