Update DuckDuckGo onion site URL in search preferences and onboarding

See user feedback here: #40509 (closed)

We need to update DuckDuckGo's onion address for both the default search engine option and in the onboarding tour (on the Circuit Display and Onion Services slides) with their new V3 onion site: https://duckduckgogg42xjoc72x3sjasowoarfbgcmvfimaftt6twagswzczad.onion (note: needs verified with DuckDuckGo)

Apologies if there's already a ticket for this – I couldn't find one.