Sync epic work items to legacy epic by default

What does this MR do and why?

Sync epic work items to legacy epic by default

Sync work items to their legacy epic by default and remove the sync_work_item_to_epic feature flag.

This feature flag still has no impact as we block creating and modifying epic work items with other feature flags (namespace_level_work_items and synced_epic_work_item_editable).

This FF is already enabled on https://gitlab.com/gitlab-org/plan-stage/ if you want to test it on prod.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Feature.enable(:synced_epic_work_item_editable)
Feature.enable(:namespace_level_work_items)

1. Go to the epics page
2. Create an epic work item (it creates a work item since namespace_level_work_items is enabled)
3. Modify the epic work item (only possible due to synced_epic_work_item_editable)
4. Change the URL from /work_items/ to /epics and see that all changes are synced.