[ECS] [Linux ECS Optimized AMIs]: Subscribe to Linux ECS Optimized AMI notifications

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**Tell us about your request**
What do you want us to build?

Customer wants to have notifications when the Linux ECS optimized AMI is going to deprecated and it seems currently there is no way for the same. however, steps are there for Windows based AMI as below:

[Subscribe to Windows AMI notifications
](https://docs.aws.amazon.com/AWSEC2/latest/WindowsGuide/aws-windows-ami.html#subscribe-notifications)

**Which service(s) is this request for?**
ECS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
Customer haven't updated AMIs since long and they recently faced issue where ecs-agent being part of old ami was not getting connected, hence tasks were failing on EC2 instances launched from old AMIs. So, customer wants to get notification when the aMI is going to get deprecated and/or when new AMIs are registered/launched by AWS.

**Are you currently working around this issue?**
How are you currently solving this problem? Manually checking the AMIs released and then updating the launch configuration/template.

**Additional context**
Anything else we should know?

**Attachments**
If you think you might have additional information that you'd like to include via an attachment, please do - we'll take a look. (Remember to remove any personally-identifiable information.)

No implementation files or tests are named in the issue. Start by reviewing the linked Windows AMI notification guidance and the stated ECS AMI deprecation and release needs; done should clearly define how customers receive notifications for Linux ECS Optimized AMIs.

Written by the indexing model from the issue text.