Add announcement for System.Linq.Async deprecation

[stephentoub]

As discussed in dotnet/runtime#79782, a full LINQ implementation for IAsyncEnumerable is being included both in .NET 10 and in a System.Linq.AsyncEnumerable package available downlevel.

We should include in the System.Linq.Async package description on nuget a description of the plans to not further evolve / maintain the System.Linq.Async package and to recommend developers move to System.Linq.AsyncEnumerable instead. It'd be good to include a similar note in a README in the relevant place in this repository.

The details should also include a migration guide. Most things will "just work", but some signatures do differ between the packages.

dotnet/docs#44886

cc: @idg10, @HowardvanRooijen

[eiriktsarpalis]

Do we need an analyzer that points users to the workaround?