Support for .net6.0, netstandard2.1

[AceCoderYoda]

Add support for .net6.0 (at least) and (optional) netstandard2.1

[dnperfors]

Care to explain why this would be needed? In both cases you can reference netstandard 2.0 packages...
(Although it is rather easy to add, but adds extra work when new .net versions come out...)

[FolkerKinzel]

First of all thanks for this excellent library.

Support for netstandard2.1 would allow to support nullable reference types.

With object o = default(OneOf<string, int>).Value; the non nullable object o is null without any compiler warning.

(Don't fear to support nullable reference types together with compatibility for old versions. There are nuget packages like Nullable that enable compatibility with older framework versions.)