Fix linked_work_items_feature_flag_enabled? FF check

What does this MR do and why?

Making the feature flag check account for work items that belong directly to a group and not a project

How to set up and validate locally

1. Enable the feature flag in Rails console Feature.enable(:namespace_level_work_items)
2. Create a group level work item with the following query at http://127.0.0.1:3000/-/graphql-explorer so you can fetch a work item in the next query

   mutation {
      workItemCreate(input: {namespacePath: "flightjs", title: "test group level work item", workItemTypeId: "gid://gitlab/WorkItems::Type/1"}) {
        errors
        workItem {
          id
          iid
        }
      }
    }

3. Fetch a wok item by IID and request the WorkItemWidgetLinkedItems field

   {
      group(fullPath: "flightjs") {
        name
        workItem(iid: "<iid_returned_in_previus_mutation>") {
          id
          iid
          widgets {
            type
            ... on WorkItemWidgetLinkedItems {
              blocked
              blockedByCount
              blockingCount
              linkedItems {
                nodes {
                  linkId
                  linkType
                  linkCreatedAt
                  linkUpdatedAt
                  workItem {
                    id
                  }
                }
              }
            }
          }
        }
      }
    }

   Before this change, you will get and undefined method for nil error
4. Use the workItemAddLinkedItems mutation

   mutation {
     workItemAddLinkedItems(input: { id: "<global ID returned in the initial mutation>", workItemsIds: ["<another work item global ID>"] }) {
       errors
     }
   }
   

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #425501 (closed)