Use AES-GCM added in .NET core 3

[coryjthompson]

As @tuwrraphael suggested, should look at using AES-GCM for .NET core version 3+, dropping the requirement for bouncycastle.

https://blogs.msdn.microsoft.com/dotnet/2018/12/04/announcing-net-core-3-preview-1-and-open-sourcing-windows-desktop-frameworks/