configs/releng/airootfs/etc/systemd/network/: move RouteMetric= from [DHCPv6] to [IPv6AcceptRA]

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.

configs/baseline/airootfs/etc/systemd/network/20-ethernet.network

@@ -8,6 +8,3 @@ Name=eth*
 [Network]
 DHCP=yes
 IPv6PrivacyExtensions=yes
-
-[DHCP]
-RouteMetric=512

configs/releng/airootfs/etc/systemd/network/20-ethernet.network

@@ -9,8 +9,13 @@ Name=eth*
 DHCP=yes
 IPv6PrivacyExtensions=yes
 
+# systemd-networkd does not set per-interface-type default route metrics
+# https://github.com/systemd/systemd/issues/17698
+# Explicitly set route metric, so that Ethernet is preferred over Wi-Fi and Wi-Fi is preferred over mobile broadband.
+# Use values from NetworkManager. From nm_device_get_route_metric_default in
+# https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/blob/main/src/core/devices/nm-device.c
 [DHCPv4]
-RouteMetric=512
+RouteMetric=100
 
-[DHCPv6]
-RouteMetric=512
+[IPv6AcceptRA]
+RouteMetric=100

configs/releng/airootfs/etc/systemd/network/20-wlan.network

@@ -8,8 +8,13 @@ Name=wl*
 DHCP=yes
 IPv6PrivacyExtensions=yes
 
+# systemd-networkd does not set per-interface-type default route metrics
+# https://github.com/systemd/systemd/issues/17698
+# Explicitly set route metric, so that Ethernet is preferred over Wi-Fi and Wi-Fi is preferred over mobile broadband.
+# Use values from NetworkManager. From nm_device_get_route_metric_default in
+# https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/blob/main/src/core/devices/nm-device.c
 [DHCPv4]
-RouteMetric=1024
+RouteMetric=600
 
-[DHCPv6]
-RouteMetric=1024
+[IPv6AcceptRA]
+RouteMetric=600

configs/releng/airootfs/etc/systemd/network/20-wwan.network

@@ -8,8 +8,13 @@ Name=ww*
 DHCP=yes
 IPv6PrivacyExtensions=yes
 
+# systemd-networkd does not set per-interface-type default route metrics
+# https://github.com/systemd/systemd/issues/17698
+# Explicitly set route metric, so that Ethernet is preferred over Wi-Fi and Wi-Fi is preferred over mobile broadband.
+# Use values from NetworkManager. From nm_device_get_route_metric_default in
+# https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/blob/main/src/core/devices/nm-device.c
 [DHCPv4]
-RouteMetric=2048
+RouteMetric=700
 
-[DHCPv6]
-RouteMetric=2048
+[IPv6AcceptRA]
+RouteMetric=700