dhcpcd service: order before network target

This reverts a change applied in PR #18491. When interfaces are
configured by DHCP (typical in a cloud environment), ordering after
network.target cause trouble to applications expecting some network to
be present on boot (for example, cloud-init is quite brittle when
network hasn't been configured for `cloud-init.service`) and on
shutdown (for example, collectd needs to flush metrics on shutdown).

When ordering after network.target, we ensure applications relying on
network.target won't have any network reachability on boot and
potentially on shutdown.

Therefore, I think ordering before network.target is better.