Fix nf_conntrack_ipv4 modprobe (#6988)

RedHat 8.3 merged nf_conntrack_ipv4 in nf_conntrack but still advertise 4.18
so just try to modprobe and decide depending on the success
Also nf_conntrack is a dependency of ip_vs, so no need to care about it

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>