Fix group-level work item labels widget

What does this MR do and why?

Fix group-level work item labels widget (#428038 (closed))

The errors on the group work item page are expected as we are still fixing backend/frontend problems to get work items at the group-level. This is part of viewing work items at the group-level (#393240 (closed))

Behind feature flag namespace_level_work_items, so no changelog required

Screenshots or screen recordings

Before	After
Screen_Recording_2023-10-20_at_2.33.25_pm	Screen_Recording_2023-10-20_at_2.35.37_pm

How to set up and validate locally

1. In rails console

   Feature.enable(:namespace_level_work_items)

2. Visit any work items list page for a paid subscription group such as http://127.0.0.1:3000/groups/flightjs/-/work_items
3. Create an epic
4. Click on created epic and confirm you can add/change labels

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 #428038 (closed)