netfilter: Enable NETFILTER_XTABLES_LEGACY
6.17 introduced a new CONFIG_NETFILTER_XTABLES_LEGACY symbol that is a dependency for the xtables APIs (iptables, arptables, ip6tables, ebtables) and defaults to n.
Although the respective command-line utilities switched over to the new nftables API long ago, at least libvirt may still use the iptables API. So enable this symbol to keep the xtables APIs available for now.
Closes: #1118653