Include group-level work items in IssuesFinder

What does this MR do and why?

When namespace_level_work_items is enabled, include group-level work items in IssuesFinder / WorkItemsFinder when querying for epic types.

This is the backend portion of #392812 (closed) for the epic list.

How to set up and validate locally

1. Enable namespace_level_work_items feature flag

2. Visit /groups/<path>/-/work_items to create epics

3. Test with a GraphQL query like:

   query {
     group(fullPath: "flightjs") {
       issues(types: [EPIC]) {
         nodes {
           id
           title
         }
       }
     }
   }

MR acceptance checklist

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