Added new work items feature flag

What does this MR do and why?

We decided to remove the ability to convert a markdown task list item to a work item Task for now. In this MR I moved this functionality to a new feature flag: work_items_create_from_markdown.

See https://gitlab.com/gitlab-org/gitlab/-/issues/368462#note_1033880891 for more context.

Screenshots or screen recordings

with :work_items on, on hover

Before	After

How to set up and validate locally

1. Enable the work_items feature flag

   Feature.enable(:work_items)

2. Create a checklist within an issue
3. Checklist should not have changed
4. Enable the work_items_create_from_markdown feature flag

   Feature.enable(:work_items_create_from_markdown)

5. You should now see the "Convert to Task" button when hovering on the checklist 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.