Ubuntu 26.04's openvswitch-switch declares Alias=openvswitch.service, so
enabling it leaves an /etc symlink that distro_ovs_unit read as a second,
EL-style umbrella unit. setup-ovn.sh then refused to pick one and every
ubuntu-26.04 bootstrap failed. A symlink back to openvswitch-switch.service
now counts as the Debian unit; a real or masked openvswitch.service still
does not.