Change feature flag check for hierarchy

What does this MR do and why?

When listing parent/children for a work item, we check only generic work_item feature flag. This is little bit confusing (as for creating/updating hiearchy widget we still use work_item_hiearchy FF), but the reason is that parent/child relationship can be created by "create from task" functionality and we still need to be able to list parent for the tasks created this way.

Related to #339664 and #366211 (closed)

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How to set up and validate locally

Feature.enable(:work_items)
Feature.disable(:work_items_hierarchy)

• Then you can still use "create from task" mutation for creating a Task from GFM, example query in !90037 (merged)
• And work item parent/child values should be still listed with query like this:

query {
  project(fullPath: "flightjs/Flight") {
    workItems(types: [TASK]){
      nodes {
        id
        iid
        title
        widgets {
          ... on WorkItemWidgetHierarchy {
            parent {
              id
            }
            children {
              nodes {
                id
              }
            }
          }
        }
      }
    }
  }
}

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