Move description widget update to service layer

...tations/concerns/mutations/work_items/update_arguments.rb

app/graphql/mutations/work_items/update.rb

app/graphql/mutations/work_items/update_widgets.rb

app/models/work_items/widgets/description.rb

app/services/work_items/update_service.rb

app/services/work_items/widgets/base_service.rb

.../work_items/widgets/description_service/update_service.rb

doc/api/graphql/reference/index.md

...l/types/work_items/widgets/description_input_type_spec.rb

.../requests/api/graphql/mutations/work_items/update_spec.rb

...s/api/graphql/mutations/work_items/update_widgets_spec.rb

..._items/widgets/description_service/update_service_spec.rb

...s/work_items/update_description_widget_shared_examples.rb