Disable IPv6 on Seed hosts

Only containers running edge services (e.g. Envoy or Nginx) should have
global IPv6 addresses.

Seed host has privileged access to all containers running on it. Access
to Seed hosts is a sensitive security surface that should not be
unnecessarily exposed to additional attack vectors. A globally routable
IPv6 address is not necessary when Seeds are managed from local network.

IPv6 also adds up to 5s to network initialization:
systemd/systemd#16547 (comment)

Author: angdraug

network/40-br0.network

@@ -3,3 +3,5 @@ Name=br0
 
 [Network]
 DHCP=yes
+IPv6AcceptRA=no
+LinkLocalAddressing=ipv4