Rename ee/app/services/ai/model_selection/update_service.rb

Follow-up of !210463 (comment 2871796817)

Currently the class at ee/app/services/ai/model_selection/update_service.rb is called Ai::ModelSelection::UpdateService. But since this class is desinged only for namespace feature setting it needs to be renamed accordingly to ee/app/services/ai/model_selection/Namespaces/update_service.rb and Ai::ModelSelection::Namespaces::UpdateService