BulkImports: do not export relations if no updates are made since last export
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Suggestion from https://gitlab.com/gitlab-org/gitlab/-/merge_requests/84207#note_901184388
When GitLab Migration runs it exports project/group relations (e.g. issues/mrs/labels) in order to import them at the destination project/group.
We recently made a change (https://gitlab.com/gitlab-org/gitlab/-/merge_requests/84207) to not run export if it's been recently exported (3 minutes), however, @reprazent suggested to check `MAX(updated_at) < export.created_at` and not perform relation export if no changes were made.
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