Rename MLDsa/SlhDsa SecretKey members to PrivateKey

Proprietary

Renames SecretKey to PrivateKey in MLDsa and SlhDsa post-quantum cryptography APIs to align with .NET 10 naming conventions.

OpenRewrite.Recipes.CSharp.Migration.Dotnet.Net10.MlDsaSlhDsaSecretKeyToPrivateKey
mod run . --recipe=OpenRewrite.Recipes.CSharp.Migration.Dotnet.Net10.MlDsaSlhDsaSecretKeyToPrivateKey

Install it once, then run it on every repository under the current directory. New to the Moderne CLI? Set it up.