Remove support for unsupported .NET Core versions

[formicas]

Currently supported versions are 3.1 and 2.1.

Drop the rest.

[senseibaka]

I'd suggest targeting whichever versions of .net standard cover core 2.1 and 3.1. To me it looks like that's standard 2.0 and standard 2.1...

If you agree, this ties in with issue #145

Note: supporting standard 2.0 maintains support for framework 4.6.1 (and 4.6.2, 4.7.X, 4.8.X... ?)

Refer https://docs.microsoft.com/en-us/dotnet/standard/net-standard

[bbarry]

I would suggest following the guidance here: https://docs.microsoft.com/en-us/dotnet/standard/library-guidance/cross-platform-targeting

technically netstandard2.0 does maintain support for desktop 4.6.1+, but there are issues with 4.6.1 specifically using netstandard2.0 libraries

meaning that the tag should be:

<TargetFrameworks>netstandard2.0;net461;netstandard2.1</TargetFrameworks>

I suppose that drops support for 4.6.0 but nobody should be using that anyway. So long as you are supporting netstandard2.0 (and please don't drop it any time soon, it is very useful for those of us maintaining code in libraries shared between legacy framework and .net core while trying to drag everyone to core).

unfortunately this removes the ArrayPool (available in netstandard2.1, netcoreapp1.0+)

also DbDataPermission is still not in standard.

To be able to not lose functionality on still supported platforms I guess:

<TargetFrameworks>netstandard2.0;net461;netstandard2.1;netcoreapp2.1;netcoreapp3.1</TargetFrameworks>

references:

• https://docs.microsoft.com/en-us/dotnet/api/system.buffers.arraypool-1?view=netcore-2.1
• https://docs.microsoft.com/en-us/dotnet/api/system.data.common.dbdatapermission?view=netframework-4.8