Rename update.rb to more explicit update_namespace_feature_setting_service.rb

Patrick Cyizarequested to merge
579830-rename-ee-app-services-ai-model_selection-update_service-rb into master

What does this MR do and why?

This simply rename the class ::Ai::ModelSelection::UpdateService into ::Ai::ModelSelection::UpdateNamespaceFeatureSettingService to the name explicitly state what it does inside the Duo workflow service.

Related to #579830 (closed)

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