Drop support for VS before 2019

[manfred-brands]

We currently have separate releases 2.x and 3.x, where the first is the code without the DiagnosticSuppressors.
Those were introduced in roslyn version 3.3.1
which matches Visual Studio 2019 16.3 which is 4 years old and predates .NET Core 3.1.

I expect all users, even those addressing .NET Framework to use later SDKs to take advantage of new C# language feates.

[manfred-brands]

@mikkelbu If you agree, could you take this on please?

[mikkelbu]

I agree that we should stop the 2.x releases. Looking at https://www.nuget.org/packages/NUnit.Analyzers the 2.x download numbers are a tiny fraction of the 3.x. I'm very busy at the moment, but I think I've time for this (it should be a small task).

[manfred-brands]

Depends on #638 which adds more #if NETSTANDARD code.

[manfred-brands]

At the same time I wanted to move to .NET8.0 SDK to match NUnit.
However, the corresponding updated Roslyn Analyzer now insists on separating the CodeFix from the Analyzers.
CodeFixes should only be loaded by IDE, not by the build process.

[manfred-brands]

In addition to that, unit tests fail due some other changes they made: GuOrg/Gu.Roslyn.Asserts#320

[mikkelbu]

@manfred-brands I think I'll make an attempt on dropping support for VS 2017 (and lower) for the release this weekend, and then make the release version 4, and then we can push .NET8.0 SDK and bumping Roslyn Analyzers to a future fix - i.e. splitting this issue into two issues.