feat(ecs): change the default generation to Amazon Linux v2

[hencrice]

This change make CDK consistent with the recommendataion we give customers: https://docs.aws.amazon.com/AmazonECS/latest/developerguide/ecs-optimized_AMI.html. This closes #3005.

BREAKING CHANGE: Previously, the default generation is conditionally set to Amazon Linux v1 if hwType is STANDARD. Now it always default to Amazon Linux v2.

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• Testing
  • Unit test added (prefer not to modify an existing test, otherwise, it's probably a breaking change)
  • CLI change?: coordinate update of integration tests with team
  • cdk-init template change?: coordinated update of integration tests with team
• Docs
  • jsdocs: All public APIs documented
  • README: README and/or documentation topic updated
  • Design: For significant features, design document added to design folder
• Title and Description
  • Change type: title prefixed with fix, feat and module name in parens, which will appear in changelog
  • Title: use lower-case and doesn't end with a period
  • Breaking?: last paragraph: "BREAKING CHANGE: <describe what changed + link for details>"
  • Issues: Indicate issues fixed via: "Fixes #xxx" or "Closes #xxx"
• [N/A] Sensitive Modules (requires 2 PR approvers)
  • IAM Policy Document (in @aws-cdk/aws-iam)
  • EC2 Security Groups and ACLs (in @aws-cdk/aws-ec2)
  • Grant APIs (only if not based on official documentation with a reference)
    By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

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[rix0rrr]

Probably more integ tests that need updating

[hencrice]

Probably more integ tests that need updating

Looks like unit tests (and likely integ tests) for other non-ECS services are failing. Will address them