Show icon of work item Epics in ancestor widget

What does this MR do and why?

While work item Epic db migration is in progress, change to epic icon for work item Epics instead of issue-type-epic in ancestor widget.

MR acceptance checklist

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Before	After

How to set up and validate locally

Prerequisites:

1. Enable the feature flags
   • Feature.enable(:display_work_item_epic_issue_sidebar)
   • Feature.enable(:namespace_level_work_items)
2. The namespace must have Ultimate plan.

Steps to verify:

1. Login with any user
2. Go to any Group > Epics
3. Modify the link path from /epics to /work_items like below, e.g. http://127.0.0.1:3000/groups/gitlab-org/-/epics > http://127.0.0.1:3000/groups/gitlab-org/-/work_items
4. Create Work Item Epic and open the new Epic
5. Add a child Epic
6. Open the child Epic
7. Repeat the process to create a hierarchy
8. On last Epic check the bread crumbs (Ancestry widget)
9. It should render the Epic icons

Related to #439939 (closed)