Let relay.tmpl be IPv4 only else we are forced to pass "ipv6_addr=" to
all templates. Without it, all IPv6 ORPorts end up with "None".

Instead, when we explicitly want a network with IPv6 support, use
relay-v6.tmpl which sets the IPv6 ORPort properly.

Fixese #40008



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