Update EpicsSelect dropdown to use Vuex

What does this MR do?

This MR updates ee/app/assets/javascripts/vue_shared/components/sidebar/epics_select component (introduced in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14763) to use Vuex and discard store and service classes. Visually this change should have no impact as it just updates architectural approach.

Conformity

• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content
• Tested in all supported browsers

Closes #13714 (closed)