That might be possible. We were within days of performing our upgrade when the blue/green feature became available for Postgres, so we didn't consider it for our work.
You may be able to boot up an 11.21 replica in an existing Aurora cluster as a read replica, and then failover to that replica as your primary, which would be a minimally disruptive process if your application is designed to tolerate replica failover.
From there, you could upgrade the rest of your cluster to 11.21, and then use the blue/green upgrade process for AWS. If you do, I'd love to hear about how it goes as we will definitely consider the blue/green feature next time.
