Moved feature flags for planning hierarchy

What does this MR do and why?

Tested the feature flag on plan-stage, and we're confident there is low enough risk to remove. We do, however, need to hide the task work item, as that has yet to be released. This MR also puts that token behind the :work_item feature flag.

Screenshots or screen recordings

:work_items enabled	:work_items disabled

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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 #340844 (closed)