[c#] csharp and csharp-dotnet2 needs new codegen

Description

'csharp' and 'csharp-dotnet2' codegens are obsolete to say the least. While i do like very much what other parts of swagger-codegen-cli generate (as its default conventions and infrastructure seems very robust) generators needs updates / replacement to keep up with trending http-clients and language features.

• Usage of M$-provided HttpClient is a safe pick for client implementation, no forced dependencies and optimal performance for pretty much most of stable .NET Core versions
• System.Text.Json as serializer works pretty much fine on most cases when api works in a way that swagger can describe it.
• usage of mono for building is not overly useful as dotnet is crossplatform without usage of mono right now

Swagger-codegen version

looking at 3.0.54

Suggest a fix/enhancement

Making some kind of clone for csharp-dotnet2 and refining code that generates clients and additional files would be nice.

Start by comparing the csharp and csharp-dotnet2 generators and the client and additional-file output they currently produce. Review how the generators build clients, serialize data, and depend on mono; done would require a defined replacement or refinement using HttpClient and System.Text.Json that builds across supported .NET Core versions.