v3 Onion Services: don't make IPv4 mandatory because one day we'll have IPv6 only relays

Right now it is not possible to have a relay that is IPv6 only in the public network but one day it will.

Proposal 224 makes IPv4 mandatory for link specifiers in order for a relay to extend to it. It would be wise to NOT make it mandatory and instead make it mandatory to at least have IPv4 or IPv6.

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