Real mutation for work item labels

What does this MR do and why?

Replace mock labels mutation with a real one for #339756 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Enable the flag

   Feature.enable(:work_items_mvc_2)

2. Visit any issue page, add a Task (grey box, tasks, Add, enter title)
3. Click task to open
4. Add some labels, then remove some labels

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.