Add samples for dual-stack and ipv6-only subnet creation

[jarzynskila]

Description

Fixes #

Note: If you are not associated with Google, open an issue for discussion before submitting a pull request.

Checklist

Readiness

• Yes, merge this PR after it is approved
• No, don't merge this PR after it is approved

Style

• My sample follows the rules described for Terraform in the Effective Samples style guide
• My sample follows the other requirements and best practices in the Contributing
  guide

Testing

• I have performed tests described in the Contributing guide:

  • Tests pass: terraform apply
    • Above test failed with "Cannot destroy project as deletion_policy is set to PREVENT."
    • Tested in my own project
  • Lint pass: terraform fmt check

Intended location

• Yes, this sample will be (or already is) included on cloud.google.com
  Location(s): https://cloud.google.com/vpc/docs/create-modify-vpc-networks

• No, this sample won't be included on cloud.google.com
  Reason:

API enablement

• If the sample needs an API enabled to pass testing, I have added the service to the Test setup file

Review

• If this sample adds a new directory, I have added codeowners to the CODEOWNERS file

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[snippet-bot]

Here is the summary of changes.

You are about to add 3 region tags.

• vpc/network_ipv6/main.tf:17, tag vpc_ipv6_internal
• vpc/network_ipv6/main.tf:25, tag vpc_subnet_dual_stack
• vpc/network_ipv6/main.tf:36, tag vpc_subnet_ipv6_only

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[glasnt]

Due to the amount of updates required for this sample, I've opted to edit the PR directly.

I will note some existing terraform samples on the docs page are from https://github.com/terraform-google-modules/terraform-google-network, rather than this repo, but we have the /vpc/ space for samples in this repo.

/gcbrun < this runs the CI