Investigate necessary changes for .NET 6.0

[thomaslevesque]

In .NET 6.0, Linq introduces a few new methods that could conflict with Linq.Extras methods or render them obsolete.

• Chunk: seems equivalent to Batch. Consider making Batch obsolete in .NET 6 (and maybe delegate the implementation to Chunk, which might have better performance)
• FirstOrDefault, SingleOrDefault, and LastOrDefault with defaultValue parameter: already exist in Linq.Extras, will cause conflict in .NET 6.0
• *By operators (MaxBy, MinBy, DistinctBy...): already exist in Linq.Extras, will cause conflict in .NET 6.0

Hopefully avoid reproducing the issues that happened with ToHashSet (#19, #22)...

[thomaslevesque]

Here's the plan:

• add a new net6.0 TFM
• for Linq.Extras methods that are being introduced in .NET 6.0 with the same name and signature, remove the this modifier in the net6.0 TFM. This way the methods won't cause ambiguous calls (avoiding Ambiguous call for ToHashSet #19), but they will still be callable at runtime (avoiding [Runtime Error] MissingMethodException caused by using ToHashSet method #22). And they will still be available to older target frameworks
• Batch will be kept as is, but marked obsolete in the net6.0 TFM
• The new ExceptBy and IntersectBy methods in .NET 6.0 don't actually have the same signature as the ones in Linq.Extras. They accept a sequence of TKey as the second parameter, instead of a sequence of TSource. So they don't do exactly the same thing. These methods can be kept in Linq.Extras without causing conflicts.
• Bring back the ToHashSet, Append and Prepend methods that were removed to fix [Runtime Error] MissingMethodException caused by using ToHashSet method #22, since we now have a way to keep them without breaking things.