Remove dependency on Mono

We currently rely on Mono to push NuGet packages and to create the Gremlin.Net.Template package.
To push the packages, we can now also simply use dotnet nuget push
and for dotnet templates, there seems to be a workaround that uses a dummy .csproj file which enables dotnet pack to create a valid NuGet package for a dotnet template.

So, we don't have to use Mono any more at all and we also shouldn't need to download nuget.exe any more during the build.