CE Backport: Show actual Milestone dates within tooltips for Milestones in Epics sidebar

What does this MR do?

This MR is CE port for https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8048

This MR backports following files from EE to be consistent between CE and EE;

• app/assets/javascripts/vue_shared/components/sidebar/collapsed_grouped_date_picker.vue
• app/assets/javascripts/vue_shared/components/sidebar/collapsed_calendar_icon.vue
• spec/javascripts/vue_shared/components/sidebar/collapsed_grouped_date_picker_spec.js

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/7103