Add support for IPv6 in DNS configuration

[jardahrazdera]

Extends the DNS setup script to support IPv6 alongside IPv4 for modern dual-stack networks.

Changes

• Cloudflare DNS: Added IPv6 servers 2606:4700:4700::1111 and 2606:4700:4700::1001
• Fallback DNS: Added IPv6 Quad9 servers 2620:fe::fe and 2620:fe::9
• IPv6 DNS resolution: Enabled DNSStubListenerExtra=[::1]:53 for IPv6 localhost
• Migration: Updates existing configurations for IPv6 support

Problem Solved

Fixes IPv6 DNS resolution failures on dual-stack networks like mobile hotspots and Starlink, where applications timeout trying to resolve domains via IPv6.

Testing

• Verified IPv6 DNS stub listener works on [::1]:53
• Confirmed dual-stack DNS resolution for both IPv4 and IPv6
• Migration preserves existing configurations

Fixes #1478