Remove 'work_item_status_feature_flag' feature flag

What does this MR do and why?

The MVC1 (Iteration 1 and 2) of configurable work item statuses behind the work_item_status_feature_flag feature flag was released in %18.2.

As planned, this MR removes the feature flag work_item_status_feature_flag in %18.4.

References

• #554715 (closed)
• #521286 (closed)
• 18.2 release post

Screenshots or screen recordings

Screen_Recording_2025-08-20_at_10.42.34_am

How to set up and validate locally

Ensure the functionalities introduced as part of MVC1 work as expected:

1. Configure statuses for a root namespace, including adding new statuses, updating existing ones, and setting default statuses.
2. Set a status for supported work items (issues and tasks) using the status widget or the /status quick action.
3. Open, close, reopen, or mark work items as duplicates, and confirm that the correct default status is applied.
4. Bulk edit work item statuses.
5. Verify that the status badge appears on issues and tasks in supported views, such as legacy issue lists, work item lists, user dashboard issue lists, advanced search result lists.
6. Filter by status in supported lists.
7. Create legacy issue board lists for a given status, and ensure issues on boards can be filtered correctly by status.

MR acceptance checklist

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