MAINTAINERS: update the IPv4/IPv6 entry and add Ido Schimmel

MAINTAINERS: update the IPv4/IPv6 entry and add Ido Schimmel

The IPv4/IPv6 and routing code is not very well separated from
the TCP/UDP code. Scope it down properly by providing a more
accurate file list, instead of net/ipv4/ and net/ipv6/

Now that the entry is more accurately representing layer 3
and routing merge in the nexthop entry into it.

Add Ido Schimmel as a co-maintainer, Ido's git history speaks
for itself.

Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20260428203924.1229169-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>