Add .NET 6.0 TFM

[thomaslevesque]

Fixes #24

• Add a new net60 TFM
• Change methods that conflict with new .NET 6.0 methods, so that they're no longer extension methods, and mark them as obsolete (in the net60 TFM only). Affected methods:
  • DistinctBy
  • UnionBy
  • Min and Max overloads that accept an IComparer<T>
  • MinBy and MaxBy
  • FirstOrDefault, LastOrDefault, and SingleOrDefault overloads that accept an explicit default value
• Make Batch obsolete in the net60 TFM (.NET 6.0 introduced Chunk, which does the same thing)
• Reintroduce Append, Prepend and ToHashSet, using the same approach. (also reintroduce the net471 and net472 TFMs)