Update Debian repository onion to v3

In the Support entry "Can I use APT over Tor?", there are instructions to use the v2 onion service of deb.torproject.org. Although it's still valid, we should promote the new v3 onion service, which is now available. According to onion.torproject.org, the new v3 onion is:

deb.torproject.org

http://sdscoq7snqtznauu.onion/
http://apow7mjfryruh65chtdydfmqfpj5btws7nbocgtaovhvezgccyjazpqd.onion/