title: Can not upgrade Aurora RDS Postgres version from 12 to 13
body: <p>I am trying to upgrade Aurora RDS Postgres cluster db.r5.xlarge from version 12.7 to 13.3.
  I choose Engine version 13.3, default DB cluster parameter group and DB parameter group default.aurora-postgresql13 and choose 'Apply immediately'.</p>
  <p>Received error message:</p>
  <blockquote>
  <p>We're sorry, your request to modify DB cluster clone-cluster has failed.
  Cannot modify engine version because instance clone-cluster is running on an old configuration.
  Apply any pending maintenance actions on the instance before proceeding with the upgrade</p>
  </blockquote>
  <p>There are no any pending maintenance actions showing in AWS RDS console and I have no idea what configuration they mean.</p>
  <p>We are on a free basic support plan no so we cannot get help from AWS. Can anyone please suggest if there's a way to upgrade the whole cluster at once?</p>