Net/DNS/Publicdns: update the IPv6 range that we use to recreate route endpoint for control D

In this commit I updated the Ipv6 range we use to generate Control D DOH ip, we were using the NextDNSRanges to generate Control D DOH ip, updated to use the correct range.

Updates: #7946
Signed-off-by: Kevin Liang <kevinliang@tailscale.com>
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