Update "convert to task" feature flag

What does this MR do and why?

This MR updates the feature flag for the "convert issue description checklist item to task" feature from work_items_mvc_2 to work_items_mvc

/cc @donaldcook

Screenshots or screen recordings

How to set up and validate locally

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 #377307 (closed)