Log warning when connecting to soon-to-be-deprecated v2 onions.

changes/ticket40373

+  o Minor features (onion services):
+    - Add warning message when connecting to soon-to-be-deprecated v2 onions.
+      Closes ticket 40373.

src/core/or/connection_edge.c

@@ -2582,6 +2582,16 @@ connection_ap_handshake_rewrite_and_attach(entry_connection_t *conn,
     tor_assert(addresstype == ONION_V2_HOSTNAME ||
                addresstype == ONION_V3_HOSTNAME);
     tor_assert(!automap);
+
+    if (addresstype == ONION_V2_HOSTNAME) {
+      log_warn(LD_PROTOCOL,
+               "Warning! You've just connected to a v2 onion address. These "
+               "addresses are deprecated for security reasons, and are no "
+               "longer supported in Tor. Please encourage the site operator "
+               "to upgrade. For more information see "
+               "https://blog.torproject.org/v2-deprecation-timeline");
+    }
+
     return connection_ap_handle_onion(conn, socks, circ, addresstype);
   }