dhcp: fix ignoring addresses with DHCPv6 otherconf (O flag)

With O flag (otherconf mode), don't add the IPv6 addresses to the
collected lease.

An alternative would be to add it initially, but ignore it when
merging the configuration in NML3Cfg. The idea of that would be that if
the mode switches from otherconf to managed, that we already have the
address. However, depending on the mode we already make different DHCPv6
requests (at least for internal client). That means, if the mode changes
we anyway cannot just use the previous lease, because it might not
contain all the information. So it seems better to ignore the address

Fixes: 58287cbc ('core: rework IP configuration in NetworkManager using layer 3 configuration')

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/issues/ ## 953
https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/ ## 1220
57 jobs for th/dhcpv6-otherconf-ignore-addr in 30 minutes and 43 seconds (queued for 4 seconds)