units: Shut down networkd and resolved on switch-root

units: Shut down networkd and resolved on switch-root

Let's explicitly order these against initrd-switch-root.target, so
that they are properly shut down before we switch root. Otherwise,
there's a race condition where networkd might only shut down after
switching root and after we've already we've loaded the unit graph,
meaning it won't be restarted in the rootfs.

Fixes #27718