relay: Change router_can_extend_over_ipv6() to look at configured port

In routerconf_find_ipv6_or_ap(), we check if the returned ORPort is internal
but not for listening. This means that IPv6 [::] is considered internal.

Thus, we can't use it, we have to look directly at the configured address and
port and if they are valid, we do consider that we have a valid IPv6 ORPort
and that we can thus extend in IPv6.

Related #33246

Signed-off-by: David Goulet <dgoulet@torproject.org>