Move AssignIpv6AddressOnCreation from task to template

[nikimanoledaki]

Description

Closes #4363

This PR removes the workaround where we had a post-creation Task to enable AssignIpv6AddressOnCreation on public subnets. The issue was that it could not be set alongside MapPublicIpOnLaunch (which is only set for public subnets) due to a CF bug.

Instead, in this PR, this is done as part of the CF template.

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the userdocs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes
• (Core team) Added labels for change area (e.g. area/nodegroup) and kind (e.g. kind/improvement)

BONUS POINTS checklist: complete for good vibes and maybe prizes?! 🤯

• Backfilled missing tests for code in same general area 🎉
• Refactored something and made the world a better place 🌟

[nikimanoledaki]

Tested the IPv6 suite:

[0] Ran 1 of 1 Specs in 2080.985 seconds
[0] SUCCESS! -- 1 Passed | 0 Failed | 0 Pending | 0 Skipped
[0] --- PASS: TestIPv6 (2080.99s)
[0] PASS