I could not find how you prevent downgrade attack would you please help me?

There is an issue with utls in Iran and we are trying to investigate that (see XTLS/Xray-core#1919 )

I can not see it in your code
https://github.com/refraction-networking/utls/blob/master/handshake_client_tls13.go

However it is clearly visible in https://github.com/golang/go/blob/ee522e2cdad04a43bc9374776483b6249eb97ec9/src/crypto/tls/handshake_client.go#L203