Ladybird Replace DNT with GPC

Global Privacy Control aims to be a replacement for Do Not Track. DNT ended up not being a great solution, as it wasn't enforced by law. This actually resulted in the DNT header serving as an extra fingerprinting data point.

GPC is becoming enforced by law in USA states such as California and Colorado. CA is further working on a bill which requires that browsers implement such an opt-out preference signal (OOPS):
https://cppa.ca.gov/announcements/2025/20250911.html

This patch replaces DNT with GPC and hooks up the associated settings.