Re-order RootCause labels and add new DB-Migration

Why is this change being made?

Adding a new ~RootCause::DB-Migration" label to help us analyse problems resulting from migrations - https://gitlab.com/gitlab-org/database-team/team-tasks/-/issues/225#note_791356758

Since we now have quite a few RootCause labels I've also re-ordered the list into alphabetical order.

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