up method
Implementation
@override
Future<int> up() async {
  final existing = await queryApplied();
  final unapplied = await validateApplied(migrations, existing);
  for (int i = 0; i < unapplied.length; i++) {
    final migration = unapplied[i];
    logger.info('applying migration: ${migration.version}');
    await apply(migration);
  }
  return unapplied.length;
}