file: /proc/sys/net/ipv6/conf/interface/force_mld_version
variable: net.ipv6.conf.interface.force_mld_version
Official reference

0 - (default) No enforcement of a MLD version, MLDv1 fallback allowed 1 - Enforce to use MLD version 1 2 - Enforce to use MLD version 2

Nb: per interface setting (where “interface” is the name of your network interface); “all” is a special interface: changes the settings for all interfaces.