Drop support for Visual Studio 2017.

[brian-reichle]

I would like to drop support for Visual Studio 2017.

This means:

• Updating the tests to net472 (from net46)
• Updating the analyzer assemblies to netstandard2.0 (from netstandard1.3)
• Updating Roslyn to 3.x (from 2.10)

I was thinking of updating Roslyn as far as we can while still supporting VS 16.4 (which is apparently still supported until October), but the only VS <--> Roslyn version mapping I could find seems to have given up with VS 2019. It should still be at least 3.3 though, which should allow us to address some nullability issues.

[yaakov-h]

Some brief poking around at the Visual Studio installer suggests that VS2019 v16.4.23 ships with Roslyn "3.4.1.2012008" with assembly versions 3.4.0.0.

Roslyn tags unfortunately don't confirm this as there don't seem to be any tags for any versions between 3.3 and 3.6.

The Microsoft.CodeAnalysis NuGet packages for v3.4.0 shipped on 3rd December 2019, the same date as VS2019 v16.4.

So I think we should be safe to target Roslyn <= 3.4.0.

[yaakov-h]

It's October now, but I can't tell if that means that support has been dropped now, or if it only gets dropped in another 30 days time.