Update Onionoo specification to protocol version 5.1

Onionoo protocol version 5.1 will include two changes (legacy/trac#24494 (moved) and legacy/trac#25241 (moved)) that we need to specify. Also, we'll have to announce protocol version 6.0 (legacy/trac#25332 (moved)) that will be released 1 month after releasing 5.1-1.11.0.

Let's discuss the necessary changes to metrics-web in this ticket. I'll work on a branch.