With "client" I meant the Windows machine that you run that netsh command on.
That command only exists on MS OS'es.
And that MS client having Google's IPv6 addresses configured for DNS probably means all your clients have them configured.
Below command run on that MS machine should also confirm having IPv6 DNS addresses configured:
ipconfig /all
Those IPv6 RA's are coming from your router and if you cant change or disable IPv6 RA DNS, only thing left is configuring all your devices with static DNS IP details manually.
Most affordable consumer routers allow to configure specific IPv6 settings.
For Pi-hole the only thing important is that the router supports advertising IPv6 ULA prefixes via IPv6 RA like depicted in below screenshots:
And that you can actually change DNS servers that are advertised via IPv6 RA (either for IPv6 SLAAC or DHCPv6 address assignment).
EDIT: This guy explains allot of those details nicely: