Update configs/releng/airootfs/etc/systemd/network/20-{ethernet,wireless}.network

• Match the device type instead of the interface name. Type=wlan should have always worked and Type=ether works since https://github.com/systemd/systemd/commit/ef62949a23a2d6dc722659cc527f6e0006135dc9 (systemd v245).
• Replace [DHCP] section with [DHCPv4]/[DHCPv6]. systemd split the sections. [DHCP] still works for both IPv4 and IPv6, but it should be better to update it anyway.