Update SignalR client to Target .Net Standard 2.1

[mikaelm12]

We are currently cross targeting to netcoreapp3.0 for IAsyncEnmerable support. It's included in .Net Standard dotnet/standard#1017 we should move to cross target that instead

https://github.com/aspnet/AspNetCore/blob/507a765dfb078f446c445318e7b55ad9f7f5cbe0/src/SignalR/clients/csharp/Client.Core/src/Microsoft.AspNetCore.SignalR.Client.Core.csproj#L5

[analogrelay]

This bit of code will cause some problems (and may already be a problem with our current cross-targeting):

https://github.com/aspnet/AspNetCore/blob/efa9a4e530b2e9ebafe672a78b5ecae2e346d19f/src/SignalR/clients/csharp/Http.Connections.Client/src/HttpConnection.cs#L587-L605

The first problem is that right now it means that if you are running the SignalR 3.0 Client on .NET Core 2.1 (a supported scenario since we target .NET Standard 2.0) then we will no longer use WebSockets on Windows 7, despite there being a managed implementation.

The second problem is that if we cross target netstandard2.1 instead of netcoreapp, we lose the ability to do this at all (via #if). We may need to do runtime detection here instead. There's no clean API for that though: https://github.com/dotnet/corefx/issues/17452

[BrennanConroy]

Moving to preview7