fix(BA-6897): make pre-constraint RBAC backfill upgrades skip existing rows#12883

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fix/BA-6896-rbac-backfill-downgrade-data-losslablup/backend.ai:fix/BA-6896-rbac-backfill-downgrade-data-lossfrom

fix/BA-6897-rbac-backfill-upgrade-idempotencylablup/backend.ai:fix/BA-6897-rbac-backfill-upgrade-idempotencyCopy head branch name to clipboard