[REQ] [donet-core] Support for dotnet core 3.1

Created by: hpollak

It leads not in a unresolveable Problem to use csharp-netcore to generate client-code for .netcore 2.x but Dotnet-core 3.1 has LTS. You can port the generated code to 3.1 but 3.1 should supported by the generator. I generated the Code from kubernetes.json - Swagger-File.

It works after a view corrections, but there are a lot of warnings (CS0472) caused by the null-checks, the functions use types i.e. bool and not bool? so its per definition not possible to be null ( this checks can be removed )

the project files contains allso the targetframework in the PropertyGroup of the SDK. Can be changed to netcoreapp3.1

So it would be nice to have a generation-option for netcoreapp3.1.