Breaking changes in dependencies / .NET Standard 2.0 support

[skolima]

Premailer in version 2.1.0 increased the package target version, but set it to known-to-be-broken .NETFramework4.6, instead of suggested .NETTarget4.6.1. Previous target was .NETFramework4.5, which made 4.6+ targets to correctly resolve to .NETStandard2.0.

This also triggers the same buggy dependency chain issue in AngleSharp, as it has the same issue AngleSharp/AngleSharp#831

MSDN docs, suggesting .NETFramework4.6.1 as minimal target https://docs.microsoft.com/en-us/dotnet/standard/net-standard

This results in non-deterministic restores during the build, because of bugs in .NETFramework4.6 support for .NETStandard2.0, which will sometimes lead to runtime binding failures.

This is the reason e.g. Dapper declares its dependencies as it does https://www.nuget.org/packages/Dapper/2.0.30