Support netstandard2.1

[PsykerUdot]

No description provided.

[BobLd]

@PsykerUdot can you explain the need to add netstandard2.1, especially on top of netstandard2.0?

[PsykerUdot]

@PsykerUdot can you explain the need to add netstandard2.1, especially on top of netstandard2.0?

Unity (www.unity.com) supports only netstandard2.1. Using libraries built against netstandard2.0 can cause dependencies conflict (as Unity itself provides system libraries of certain versions).

[cremor]

I don't need .NET Standard 2.1 support (and I don't understand how it would make a difference at runtime), but the way it was done here is not enough to really enable it.

• UglyToad.PdfPig.Package.csproj wasn't updated, so the NuGet package doesn't contain the new target.
• There are many Condition attributes in various csproj files that check for netstandard2.0. I assume those should have been updated to also check for netstandard2.1.