Fix weight widget for group level work items

What does this MR do and why?

issue_weights feature availability was checked only against an issues's project. We should check against a group when the issue is directly associated with one

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. update the weight on the newly created work item

   mutation {
      workItemUpdate(input: {id: "<gid_from_previous_step>", weightWidget: {weight: 3}}) {
        errors
        workItem {
          workItemType {
            name
          }
          widgets {
            ... on WorkItemWidgetWeight {
              weight
            }
          }
        }
      }
    }

In the response before this change you will get no weight widget, nor you will see that the value has been updated in the DB using the rails console.

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 #429598 (closed)