Use start and due date attributes when listing iterations

...ssets/javascripts/boards/components/board_list_header.vue

...ts/javascripts/boards/components/board_add_new_column.vue

...ts/javascripts/boards/graphql/board_list.fragment.graphql

...pts/iterations/components/iteration_cadence_list_item.vue

...ts/javascripts/iterations/components/iteration_period.vue

ee/app/assets/javascripts/iterations/utils.js

...ets/javascripts/sidebar/components/iteration_dropdown.vue

.../sidebar/components/iteration_sidebar_dropdown_widget.vue

...ts/javascripts/sidebar/queries/iteration.fragment.graphql

ee/spec/features/issues/issue_sidebar_spec.rb

...c/frontend/boards/components/board_add_new_column_spec.js

ee/spec/frontend/boards/components/board_list_header_spec.js

ee/spec/frontend/boards/mock_data.js

...iterations/components/iteration_cadence_list_item_spec.js

...ec/frontend/sidebar/components/iteration_dropdown_spec.js