12.3.0 register services error，thks

[dean1979]

RegisterServicesFromAssembly(Assembly.GetExecutingAssembly());

[zachpainter77]

This issue has been tracked here: #1047

And a PR has been created fixing this issue here: #1048

[zachpainter77]

The fix for this is to:

1. Rollback to the previous version of mediator
   OR
2. Add configuration to tell MediatR to not try to register generic types.

builder.Services.AddMediatR(config => {
    config.TypeEvaluator = x => !x.ContainsGenericParameters;
    config.RegisterServicesFromAssemblies(assemblies);
});

[zachpainter77]

This is a work around until the bug fix has been merged to master.