CHG : Remove explicit support for net40 and net47

[ebfortin]

net47 is supported defacto through netstandard2.0. See reference : https://docs.microsoft.com/en-us/dotnet/standard/net-standard

[tmilnthorp]

net47 is supported defacto through netstandard2.0. See reference : https://docs.microsoft.com/en-us/dotnet/standard/net-standard

Yes but there are issues in the transient reference to the ValueTuple types depending on the runtime. See #960

[tmilnthorp]

And #959

[angularsen]

@tmilnthorp can you give latest 5.0 nuget a try and see if this is still a problem now that we removed all targets but netstandard2.0?