Expose work_item_id for legacy epics

What does this MR do and why?

Expose work_item_id for legacy epics

Exposes the work_item_id for legacy epics to make it easier to map the legacy epic IDs to the new work item.

Changelog: added EE: true

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. GraphQL http://gdk.test:3000/-/graphql-explorer

query {
  group(fullPath:"gitlab-org") {
    epics {
      nodes{
        id
        workItemId
      }
    }
  }
}

2. REST API

curl --header "PRIVATE-TOKEN: PAT \
   "http://gdk.test:3000/api/v4/groups/gitlab-org/epics" | jq  '.[] | {id: .id, work_item_id: .work_item_id}'

{
  "id": 9,
  "work_item_id": 620
}
{
  "id": 8,
  "work_item_id": 619
}
{
  "id": 6,
  "work_item_id": 617
}
{
  "id": 7,
  "work_item_id": 618
}
{
  "id": 10,
  "work_item_id": 621
}
➜  gitlab git:(nd/work-item-id-for-epics) ✗

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