Update Cross Platform Cryptography Guide for .NET 10

[vcsjones]

Describe the issue or suggestion

.NET 10 is going to have some substantial changes to cryptography. We should update https://learn.microsoft.com/dotnet/standard/security/cross-platform-cryptography to reflect them.

• Introduction of PQC
• Dropping support for OpenSSL on macOS

[vcsjones]

I will do this as we get closer to RC1 and some more concrete information is available.