Fix updated_at column desync

What does this MR do and why?

Related to #454822 (closed)

Ensure the update_at timestamp is kept in sync between epic and associated work items after creating links.

Usually, system notes are created after the create/destroy transaction is finished which would make this column of out sync, so we now call #sync_work_item_updated_at on the epic that was modified to updated its associated work item column.

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.

✅