[Onion Services] Replace mailing list tor-onions link to the Tor Forum Onion Services category

Here: https://community.torproject.org/onion-services/

"Are you interested in learning more about onion services? Join our tor-onions mailing list to speak with other onion service operators. "

We should consider changing the mailing list link to the Tor Forum: https://forum.torproject.org/c/support/onion-services/16.

(cc @rhatto)