Warn about deprecated v2 .onion

As sent to tor-dev (https://lists.torproject.org/pipermail/tor-dev/2020-June/014365.html) v2 onion services will be deprecated starting with tor 0.4.4.x (around Sep 15, 2020).

We should include a proper deprecation warning in Tor Browser, too, likely with Tor Browser 10 which is supposed to get out on Sep 22, 2020).