End support for NETSTANDARD 1.6

[cijothomas]

.NET Core 1.1 will end support on June 27 2019.
https://devblogs.microsoft.com/dotnet/net-core-1-0-and-1-1-will-reach-end-of-life-on-june-27-2019/

This repo (and other repos), have lot of conditional compilations for NETSTANDARD1_6,. Proposing to remove all NETSTANDARD 1_6 or lower support.

Once .net core 3.0 is GAed, we'll need to add 3.0 tests as well, so removing 1.1 could be clubbed together.

[TimothyMothra]

It looks like NETStandard.Library v1.6.1 might be bringing in System.Net.Http v4.3.0 which is out of compliance.

[TimothyMothra]

i'm closing this parent issue as we're tracking this effort in smaller issues.