Fix AMI cleanup workflow to deregister AMI before deleting snapshots

[fsatsuki]

Description

This PR fixes the Step Functions state machine for AMI cleanup (issue #171). The current implementation attempts to delete snapshots in parallel with deregistering AMIs, which causes errors since snapshots that are in use by AMIs cannot be deleted.

Changes

• Modified the state machine in asl.json to ensure that AMIs are deregistered before attempting to delete their associated snapshots
• Replaced the parallel execution with a sequential workflow:
  1. Delete Image Builder Image
  2. Deregister AMI
  3. Delete associated snapshots

This change ensures that snapshots are only deleted after the AMI has been successfully deregistered, which should resolve the error:

"Error": "Ec2.Ec2Exception", 
"Cause": "The snapshot snap-0xxxxxxxxxxxx is currently in use by ami-xxxxxxxxxxxx"

Fixes #171