Remove dependency on System.Linq.Async

[jcracknell]

System.Linq.Async appears to be used exclusively to provide ToListAsync, which is ridiculous as it is trivial to implement.

It also introduces extension methods into the System.Linq namespace which conflict with common EntityFrameworkCore usage patterns, as DbSet<T> implements both IAsyncEnumerable<T> and IQueryable<T>. This most often manifests itself as a conflict between:

System.Linq.AsyncEnumerable.Where<T>(this IAsyncEnumerable<T>, Func<T, bool>)

and

System.Linq.Queryable.Where(this IQueryable<T>, Expression<Func<T, bool>>)

You can watch both Microsoft teams blame each other here:

https://github.com/dotnet/reactive/issues/1057 https://github.com/dotnet/efcore/issues/18124

[jcracknell]

Hold up, I didn't namespace the extension. I'll push an update.