Update Unicode version for .Net Core

[svick]

The article on String contains the following table:

Notice that it says that all versions of .Net Core use Unicode version 8.0. But based on dotnet/coreclr#20983, I believe .Net Core 3.0+ uses Unicode version 11. And based on dotnet/runtime#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.