ecs: the default Amazon Linux generation should be v2

[hencrice]

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• I'm submitting a ...

  • 🪲 bug report
  • 🚀 feature request
  • 📚 construct library gap
  • ☎️ security issue or vulnerability => Please see policy
  • ❓ support request => Please see note at the top of this template.

• What is the current behavior?
  Current default generation is Amazon Linux v1

• What is the expected behavior (or behavior of feature suggested)?
  The expected behavior is that by default, CDK should select Amazon Linux v2 instead of Amazon Linux v1.

• What is the motivation / use case for changing the behavior or adding this feature?
  This change make the CDK consistent with the recommendation we give customers: https://docs.aws.amazon.com/AmazonECS/latest/developerguide/ecs-optimized_AMI.html

• Please tell us about your environment:
  N/A

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. associated pull-request, stackoverflow, gitter, etc)
  N/A