Minimum schema version of Drupal 7 required for upgrade to Backdrop.
Upgrades from Drupal 7 to Backdrop require that Drupal 7 be running the most recent version, or the upgrade could fail. We can't check the Drupal 7 version once the update process has begun, so instead we check the schema version of system.module in the system table.
includes/ update.inc, line 18
- Backdrop database update API.