I've just upgraded my ASP web API project from .Net core 2.0 to 3.0. I was using

     services.AddMvc()
             .AddJsonOptions(options =>options.SerializerSettings.ContractResolver 
                                       = new DefaultContractResolver());

previously to ensure lower-casing of the serialized JSON.

After the upgrade to 3.0 I get this error:

  
Error CS1061  'IMvcBuilder' does not contain a definition for
  'AddJsonOptions' and no accessible extension method 'AddJsonOptions'
  accepting a first argument of type 'IMvcBuilder' could be found (are
  you missing a using directive or an assembly reference?)

According to AddJsonOptions for MvcJsonOptions in Asp.Net Core 2.2 the AddJsonOptions extension method is/was provided by the Microsoft.AspNetCore.Mvc.Formatters.Json nuget package. I have tried installing/reinstalling this but still can't resolve the method. Interestingly, intellisense only shows Microsoft.AspNetCore.Mvc.Formatters.Xml when I try to add the using statement even though I added the Json nuget package.

Any ideas what is going on? The documentation for AddJsonOptions only goes up to .Net 2.2 so perhaps the method has been deprecated in 3.0 in favor of some other configuration mechanism?