Roll out backend work item task list toggling

What does this MR do and why?

Rolls out the work_item_task_list_toggle FF, enabled across all of GitLab.com for several days now. The backend now does task list toggling in work item descriptions (same as we already have in merge requests), preventing unexpected reverts of unrelated concurrent description edits.

See:

• Toggling an issue task item checkbox updates th... (#582358 - closed)
• [FF] `work_items_task_list_toggle` -- toggle ta... (#605619 - closed)

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.