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).

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information