MAINTAINERS: step down as netfilter maintainer

I do not feel that I'm up to the task anymore.

I hope this to be a temporary emergeny measure, but for now I'm sure this
is the best course of action for me.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240319121223.24474-1-fw@strlen.de


Signed-off-by: Paolo Abeni <pabeni@redhat.com>