Notification of new AMI release via SNS topic

### Summary
Is there a public SNS topic to subscribe to receive updates when a new ECS Optimized AMI is available?

### Description
I had a vague memory of this and had to dig back to find this old announcement: https://aws.amazon.com/about-aws/whats-new/2017/03/introducing-notifications-for-new-amazon-ecs-optimized-ami-releases/
But the link in the announcement seems to no longer exist.

We are trying to build some automation around new releases of ECS Optimized AMI versions where we then take the AMI and build our own customized AMI using it as a base. Having an SNS topic would be very helpful for this.

Start by reviewing the linked 2017 announcement and checking the current ECS Optimized AMI release information. Determine whether a public SNS topic is available and define the supported notification mechanism for new AMI releases. Done means the availability and subscription details are documented or the requested capability is clearly scoped.

Written by the indexing model from the issue text.