Minimum schema version of Drupal 7 required for upgrade to Drupal 8.
Upgrades from Drupal 7 to Drupal 8 require that Drupal 7 be running the most recent version, or the upgrade could fail. We can't easily check the Drupal 7 version once the update process has begun, so instead we check the schema version of system.module.
const REQUIRED_D7_SCHEMA_VERSION = '7069';