Use PackageReference instead of .nuspec file

We need to update the project file to include all the metadata that we have into the `.nuspec` file, in order to remove the duplication with the releases notes, that way we only need to update the `ChangeLog.md` for new releases. Probably, we need to update the `build.cake` file.

https://docs.microsoft.com/en-us/nuget/reference/msbuild-targets#pack-target