BulkImports: do not export relations if no updates are made since last export
<!--IssueSummary start--> <details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=357969) </details> <!--IssueSummary end--> 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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