Fix group work item autocomplete source path

What does this MR do and why?

Fix group work item autocomplete source path (#435403 (closed))

Behind feature flag namespace_level_work_items

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screen_Recording_2024-01-26_at_3.53.14_pm

How to set up and validate locally

1. In rails console enable the feature flag

   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. Open the epic and confirm @ autocomplete works in comments/description

Related to #435403 (closed)