How can this be accomplished?
Create an Auto Scaling target tracking scaling policy to gradually move traffic from the old version to the new one
Change the Application Load Balancer to a Network Load Balancer, then add both Auto Scaling groups as targets
Use an Amazon Route 53 weighted routing policy to gradually move traffic from the old version to the new one
Deploy Amazon Redshift to gradually move traffic from the old version to the new one using a set of predefined values
Explanations:
An Auto Scaling target tracking scaling policy is used for scaling the number of instances in an Auto Scaling group, not for shifting traffic between different versions of an application.
Changing to a Network Load Balancer would not help with gradually shifting traffic between application versions, as it doesn’t support routing based on version-specific conditions.
Amazon Route 53 weighted routing allows traffic to be split based on weights assigned to different resources, making it suitable for gradually shifting traffic between the old and new versions of the application.
Amazon Redshift is a data warehousing service, not a tool for shifting traffic between versions of an application. It is irrelevant to the described task.
I outline that the answer is:
Use an Amazon Route 53 weighted routing policy to gradually move traffic from the old version to the new one