Modernized .NET SDKs on .NET 10

Both our .NET SDKs are now on .NET 10. Fetch SDK 3.1 fixes GraphQL locale handling, makes caching thread-safe and gives clearer errors. Management SDK 2.0 moves up from .NET 6 and covers nearly the whole Management API, with full async and cancellation, typed errors and support for the USA 2 region. It's a breaking change from 1.x, which keeps getting fixes; see the migration guide.