Downgrade to netstandard2.0

[erikvdv1]

The iSHARE library requires netcoreapp3.1 which is not compatible with the .NET Framework (non-core). By downgrading to netstandard2.0 a much wider compatibility range can be achieved.

The code changes are minimal and have negligible impact on the library.

If you accept this pull request, it would be nice to push a new NuGet package.

[erikvdv1]

The tests fail due to a missing capability_url for EU.EORI.NL000000000 in the response. It should be https://scheme.isharetest.net/capabilities.

[erikvdv1]

@zsalkus Do you have time to look at the failing test and this PR?

[zsalkus]

@zsalkus Do you have time to look at the failing test and this PR?

Anything below Core 3.0 doesn't support C# 8 and it was added as a language version to the project with a version which doesn't support it. annotations is not supported in .NET standard either, it needs C# 8 compiler.

[erikvdv1]

@zsalkus Do you have time to look at the failing test and this PR?

Anything below Core 3.0 doesn't support C# 8 and it was added as a language version to the project with a version which doesn't support it. annotations is not supported in .NET standard either, it needs C# 8 compiler.

The aim of this PR is to support .NET Standard 2.0 and increase compatibility reach with minimal effort. Since C#8 features are used throughout the code base, I added it to the project file. It is true that officially this isn't support but if staying away from specific features (such as IAsyncEnumerable and default interface methods) it is perfectly fine. The alternative is to rewrite the C#8 code.

If you disagree, feel free to close this PR.

[erikvdv1]

The alternative is to rewrite the C#8 code.

I see you did just that in #2 👍 Thanks!