Rename tmp_epic_id to legacy_epic_id

What does this MR do and why?

Issue: #446071 (closed)

The tmp_epic_id on the issues table was initially planned to be only used to for not duplicating epic work items on the issues table.

However, as part of the epic to work item migration work we will temporarily query data that either belongs to the Epic or the synced Work Item (in the issues table).

For example, when we want to query all notes of a work item, we will query all notes that belong to the epic.id or the issue.id. While the epics table already has an issue_id, we want to prevent to JOIN the epics table all the time.

By renaming the column from tmp_epic_id to legacy_epic_id we want to indicate that the table can be used for exactly that reason.

In the future, we still want to drop the column again as it should not be necessary to query epic related data from the issues table.

Changelog: changed

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