fix: EnablePrefixForIpv6SourceNat is only applicable to NLB

[wweiwei-li]

Description

• The modify loadBalancer subnetMappings is being called on every update due to because resLBEnablePrefixForIpv6SourceNatValue is "" for ALB , even though it should prevent the call when it is.

Checklist

• Added tests that cover your change (if possible)
• Added/modified documentation as required (such as the README.md, or the docs directory)
• Manually tested
• Made sure the title of the PR is a good description that can go into the release notes

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 🌟

[k8s-ci-robot]

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This pull-request has been approved by: wweiwei-li
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