fix(ec2-alpha): update default config for Subnet's assignIpv6AddressOnCreation

[Leo10Gama]

Issue # (if applicable)

N/A

Reason for this change

The Subnet.assignIpv6AddressOnCreation attribute was being set in the L1, despite it not being necessary to set. This may cause issues with backwards migrations from V1 subnets to V2, since there will be this attribute difference across both.

Description of changes

const subnet = new CfnSubnet(this, 'Subnet', {
  ...
  assignIpv6AddressOnCreation: props.assignIpv6AddressOnCreation /* ?? false */,
  mapPublicIpOnLaunch: props.mapPublicIpOnLaunch /* ?? undefined */ // this one was just a redundant nit
}

Describe any new or updated permissions being added

N/A

Description of how you validated changes

yarn test && yarn integ-runner --update-on-failed --dry-run (dry run since it's a very minor change, and some of the integ tests for IPv6 and IPAM are finnicky)

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[aws-cdk-automation]

(This review is outdated)

[Leo10Gama]

Exemption Request: I updated the snapshots, but the integ tests themselves are fine

✅ Updated pull request passes all PRLinter validations. Dismissing previous PRLinter review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.98%. Comparing base (74cbe27) to head (74ed628).
Report is 30 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #34116   +/-   ##
=======================================
  Coverage   83.98%   83.98%           
=======================================
  Files         120      120           
  Lines        6976     6976           
  Branches     1178     1178           
=======================================
  Hits         5859     5859           
  Misses       1005     1005           
  Partials      112      112           

Flag	Coverage Δ
suite.unit	83.98% <ø> (ø)

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Components	Coverage Δ
packages/aws-cdk	∅ <ø> (∅)
packages/aws-cdk-lib/core	83.98% <ø> (ø)

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[aws-cdk-automation]

AWS CodeBuild CI Report

• CodeBuild project: AutoBuildv2Project1C6BFA3F-wQm2hXv2jqQv
• Commit ID: 74ed628
• Result: SUCCEEDED
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