Update Unicode version for .Net Core

The article on `String` contains [the following table](https://docs.microsoft.com/en-us/dotnet/api/system.string?view=netcore-3.1#strings-and-the-unicode-standard):


Notice that it says that all versions of .Net Core use Unicode version 8.0. But based on https://github.com/dotnet/coreclr/pull/20983, I believe .Net Core 3.0+ uses Unicode version 11. And based on https://github.com/dotnet/runtime/pull/33538, .Net 5 will use Unicode 13.0. Because of that, I think the table should be updated. And including information about other runtimes (Xamarin, UWP) would be nice too.