[Build] Use slnx

[sangkilc]

We will switch to slnx. The slnx support is available since 9.0.200, but the only problem is that Ubuntu repos do not support the new SDK (ref: https://developercommunity.visualstudio.com/t/dotnet-90200-not-available-via-ubuntu/10856694?stateGroup=active)

We will wait until slnx support is widely available, and switch to slnx completely.