Add .netstandard2.0 target to NSubstitute.Analyzers.CSharp

[bdovaz]

Unity only works with .NET Standard 2.0+ and doesn't get along with older packages that don't explicitly support .NET Standard 2.0.

@tpodolak Is it ok if I make a PR to change it?

https://github.com/nsubstitute/NSubstitute.Analyzers/blob/dev/src/NSubstitute.Analyzers.CSharp/NSubstitute.Analyzers.CSharp.csproj#L3

[tpodolak]

Judging by https://stackoverflow.com/a/68424149/11779643 this should not be breaking change, so go ahead. However, change it everywhere where applicable - there are more projects targeting netstadnard1.3. You might also need to check if VSIX projects are fine with that change. VISX extensions are not published but they are sometimes useful for debugging analyzers inside running VisualStudio instance