fix: internal ipv6 being used as external

[TroyKomodo]

Description

This change undoes a check for internal IPv6 addresses to be used when External IP has been requested.

Closes #4807

Checklist

• Unit tests updated
• End user documentation updated

[linux-foundation-easycla]

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• ✅ login: TroyKomodo / name: Troy (c735e76)

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[mloiseleur]

@TroyKomodo This subject has already been discussed see #4574 (comment):

We are discussing external-dns change in https://kubernetes.slack.com/archives/C771MKDKQ/p1723531094274339
and the InternalIP/ExternalIP topic was discussed in https://kubernetes.slack.com/archives/C09QYUH5W/p1723197933110369 .
sig-network tech-leads said that node InternalIP/ExternalIP has no real meaning and different cloud providers might use it differently.

FTM, we concluded it should be provider specific. Wdyt ? Does it makes sense to you ? Do you think you can rework this PR accordingly ?

[TroyKomodo]

I think it doesn't make sense to force everyone to use a specific bug-fix related to AWS not assigning Node IPs correctly. If anything the default behaviour should likely be to not include the Internal IPv6 addresses as external at all unless an annotation is specified. I also think its a fringe use case in AWS since typically there you would make use of the ELBs rather than expose a port on a host.

I think, at a minimum it should check if the IPv6 address is even routable (not in a restricted range) before attempting to use it as an external address. However IMO the best approach is to not include Internal IPv6 addresses in the external addresses unless an annotation is specified to do so.

Am willing to rework the PR to be whatever is decided the best approach is.

[TroyKomodo]

@mloiseleur any updates here?