Add work_items_project_issues_list feature flag

What does this MR do and why?

Add work_items_project_issues_list feature flag

Adds a new feature flag to control access to the work item powered project issues list. This replaces work_items_alpha which previously controlled access to the feature.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

Check the following cases are true:

work_items_alpha enabled	work_items_project_issues_list enabled	renders work items app
❌	❌	❌
✅	❌	❌
❌	✅	✅
✅	✅	✅

You can check whether the work items app is rendered by checking whether the 'new' button in the top right says 'New issue' (legacy) or 'New item' (work items).

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.

relates to &17646