Sync opening and closing of epics to work item

What does this MR do and why?

Part of #434530 (closed)

As part of our migration of moving Epics to Work Items, we also need to sync the opening and closing of epics to the work item.

If a work item exists for an epic, we now sync the state for open and close.

MR acceptance checklist

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Before	After

How to set up and validate locally

1. Enable feature flag epic_creation_with_synced_work_item
2. Go to a group -> create an epic
3. E.g. the epic is on the URL http://gdk.test:3000/groups/flightjs/-/epics/44, then open another tab with the synced work item of the epic: http://gdk.test:3000/groups/flightjs/-/work_items/44.
4. Now close the original epic and afterwards refresh the work item page. It should be closed as well.
5. Note: We explicitly don't sync the system notes of the epic.
6. Note: If you see an error when closing an epic - this is not related to this MR but a general error we have on the FE with closing/reopening epics. You can reproduce it also without my changes.

You can also check the status in the rails console:

epic = Epic.find(YOUR_EPIC_ID)
work_item = epic.work_item

epic.status 
work_item.status

epic.closed_by
work_item.closed_by

epic.closed_at
work_item.closed_at