Note that public IPv6 addresses (aka GUAs, range 2000::/3) may be subject to frequent changes, as both the IPv6 prefix as well as the interface identifier portion may change regularly (the former initiated by your ISP, the latter by things like IPv6 Privacy Extensions, e.g.).
If your router supports it, you could consider to Use IPv6 ULA addresses for Pi-hole instead (range fd00::/8).
If it doesn't, configuring Pi-hole's link-local IPv6 (range fe80::/10) could be also an option.
Use that with caution, though, as that address is only visible on the same network segment (or link ). So any L3 switches or VLANs, and devices connected through those won't be able to communicate with Pi-hole that way.
EDIT:
That file is not meant to edit manually.
Run pihole -r with Reconfigure to make your Pi-hole installation aware of your network configuration.