Set all work_item_agent_plans.ai_planning_enabled to true

What does this MR do and why?

This was originally expected to exist as an opt-in mechanism, but now it only gates the display of a widget so we need to update all records to true.

Very few records in .com (~800) so using a post deploy migration to update them all. Table doesn't have an id column, but batching will work as it defaults to primary_key which is work_item_id for this table

Related to #619376 (closed)