NEWS: announce legacy iptables/libiptc support will be dropped in v259

NEWS: announce legacy iptables/libiptc support will be dropped in v259

nftables is available since kernel 3.13 (released on 19 January 2014).
Major distributions have already provided nftables, and marked/called
iptables as deprecated or legacy.

Moreover, currently, iptables/libiptc backend does not support IPv6.

Hence, it is not necessary to keep iptables/libiptc backend anymore.
Let's drop it in the next release.

Note, fedora/centos have already disabled iptables/libiptc support since v249.