update v3 dir spec to say that v2 is obsolete
the v3 dir spec says
Caches and authorities must still support older versions of the directory protocols, until the versions of Tor that require them are finally out of commission.
which implies that the existing directory authorities are still answering v2 requests (e.g. network statuses). In fact they're not. And the caches haven't been for even longer.
dir-spec-v2.txt should probably go to the attic too.