Gate group level work item creation behind FF

What does this MR do and why?

Creating a work item that belongs directly to a group is now only possible if the namespace_level_work_items feature flag is enabled.

This should always have been released behind a feature flag since group level work items is a feature in development. Currently no way to fetch created work items at the group level, that is also under development.

THis is not a breaking change since the workItemCreate mutation is an alpha mutation. I also have confirmed we have 0 work items created at the group level in .com at the moment

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #392812 (closed)