systemd moved the option. See https://github.com/systemd/systemd/commit/8ebafba9f987c21aa5787c8767f2e390b4ec0bc5 .
Implements #123.

Document in comments why the route metrics need to be set (because of https://github.com/systemd/systemd/issues/17698 ) and use the same metric values as NetworkManager. https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/blob/main/src/core/devices/nm-device.c

Additionally remove RouteMetric from configs/baseline/airootfs/etc/systemd/network/20-ethernet.network.
There is only one networkd configuration file in baseline, meaning, there are no other routes.