Update C# client for .NET 6, get .NET client unit tests passing

[richscott]

The C#/.NET client code in client/dotnet yields a number of errors and warnings when attempting to build and run it using .NET Core 6.0. The .csproj values should be updated to use .NET 6, as well as the supporting library references to contemporary versions, including the NSwag generated Armada.Client/ClientGenerated.cs. The unit tests need some fix-ups so they start passing successfully.