JsonResult(object) causes “The collection type ‘Newtonsoft.Json.Linq.JToken’ is not supported.”

Migrate from ASP.NET Core 2.2 to 3.0

• Add a package reference to Microsoft.AspNetCore.Mvc.NewtonsoftJson

• Update Startup.ConfigureServices to call AddNewtonsoftJson:
  services.AddMvc().AddNewtonsoftJson();