RFC Final Comment Period: Shorten default TCP keepalive time

An RFC has now entered its Final Comment Period. In 14 days, the discussion will end and the proposal will either be accepted, rejected or withdrawn: https://gitlab.archlinux.org/archlinux/rfcs/-/merge_requests/51 Please visit the above link for discussion. Summary: Set our default `net.ipv4.tcp_keepalive_time` to override the extremely conservative kernel default.