Move from netstandard1.6 to netstandard2.0

[caleblloyd]

Moving from netstandard1.6 to netstandard2.0 would open a significant number of APIs including ClientWebSocket

https://docs.microsoft.com/en-us/dotnet/standard/net-standard

.NET Standard 1.6 has 13,501 of the 37,118 available APIs.

.NET Standard 2.0 has 32,638 of the 37,118 available APIs.

It would drop support for .NET Core 1.x, however maintenance ended on that June 27, 2019

[caleblloyd]

This issue is a pre-requisite for #383 and #407

[caleblloyd]

https://learn.microsoft.com/en-us/lifecycle/faq/dotnet-framework

Support for .NET Framework 4.5.2, 4.6, and 4.6.1 ended on April 26, 2022. Customers and developers must have completed the in-place update to .NET Framework 4.6.2 or later by April 26, 2022 to continue receiving technical support and security updates.

.NET Framework 4.6.2 fully supports netstandard2.0 so we can also drop support for net46 and still support all of the .NET Framework versions that Microsoft officially supports

[scottf]

See V2 client