Replace local mutation for creating a task with the real mutation

What does this MR do and why?

This MR replaces the current local mutation to create a work item and update an issue description with the real one

How to set up and validate locally

1. Enable the work items feature flag

   Feature.enable(:work_items)

2. Open an issue and add some checkboxes to the description
3. Click on three dots icon left from the checkbox and convert a task to work item

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.