Minimum schema version of Drupal 6 required for upgrade to Drupal 7.
Upgrades from Drupal 6 to Drupal 7 require that Drupal 6 be running the most recent version, or the upgrade could fail. We can't easily check the Drupal 6 version once the update process has begun, so instead we check the schema version of system.module in the system table.
define('REQUIRED_D6_SCHEMA_VERSION', '6055');