Fix fetching notes widget for group work items

What does this MR do and why?

Small policy change required to allow fetching notes created on a group level work item

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 use it 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. create a note on the newly created work item

    mutation {
      createNote(input:{noteableId: "<id_from_the_previous_step>", body: "testing note"}) {
        errors
        note {
          body
          authorIsContributor
          project {
            id
          }
        }
      }
    }

4. fetch notes for the newly created work item

   {
      workItem(id: "<id_from_the_first_step>") {
        id
        title
        archived
        project {
          id
        }
        widgets {
          ... on WorkItemWidgetNotes {
            discussions {
              nodes {
                notes {
                  nodes {
                    body
                  }
                }
              }
            }
          }
        }
      }
    }

This should return the note you created on step 2. Before this change, the last query would always return an empty array of discussions

This feature is still behind a feature flag so no changelog required

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 #428853 (closed)