Upgrade Portable.BouncyCastle C# .NET 1.8.8 or later.

[xieskysky]

Currently, the project uses

This version has a security alert
https://github.com/bcgit/bc-csharp/wiki/CVE-2020-15522

[mapelpapel]

There is an even newer version in a new repo: https://www.nuget.org/packages/BouncyCastle.Cryptography/
But I don't know if there are breaking changes.