The AWS team in charge of upgrading the images takes some time to publish them. We send the new images on every release, but they are taking longer than expected to release them. We recommend you using the EC2 instances, where the latest images are already published. You can get more information in the next link
Additionally, you can launch a new regular server in AWS Lightsail without any Bitnami installation, and manually install the latest installer on it. After that, you can migrate your application data to the new instance.