Consolidate feature flags for work items epics

Consolidate all changes required for the GA of work item epics under a single feature flag that is not dependent on the status of ​​namespace_level_work_items.

Proposal

Create a new feature flag work_item_epics and move the functionality needed from namespace_level_work_items and any other FF that cannot be removed before GA.

Introduce a new feature flag create_group_level_work_items to guard the creation of WIs at the group level with any type except Epic. This is required to keep the option disabled when work_item_epics is enabled.

List of feature flags to update

Flag	Description	Status	DRI
✅namespace_level_work_items	Work items on the group-level	default enabled: false staging: enabled globally .com: and a few groups	@egrieff
✅ synced_epic_work_item_editable	When disabled makes the work item read-only if there is a legacy epic. Can be removed once all backfills are done.	default enabled: false	@nicolasdular
work_items_rolledup_dates	Calculates the start/due dates within a hierarchy for work items	default enabled: false staging: enabled globally .com: plan-stage group	@kassio
✅ Removed sync_work_item_to_epic	synching data from epic work item to legacy epic	removed	@nicolasdular
✅ epic_and_work_item_notes_unification	to enable delegation of notes between epics and epic work item	default enabled: false staging: enabled globally .com: groups: plan-stage , issue-reproduce/product-planning-test-group and gitlab-org	@acroitor
✅ epic_and_work_item_labels_unification	to enable delegation of labels between epics and epic work item	default enabled: false staging: enabled globally com: groups: plan-stage , issue-reproduce/product-planning-test-group and gitlab-org	@acroitor
✅ display_work_item_epic_issue_sidebar	enables displaying a work item epic as the parent of a legacy issue	default enabled: false	@ramistry

Consolidated flags

Flag	Status
work_item_epics	Disabled by default, enabled in production for the group gitlab-com and gitlab-org
work_items_rolledup_dates	Globally enabled
epic_and_work_item_associations_unification	Globally enabled