rebuild fallax as ns3

fallax is an old box that we can't seem to fix on the new gnt-dal cluster (see #29974 (comment 2888671)). it's also running buster, so we need to upgrade it anyways.

let's build a new nameserver from scratch, so that we know how to do this in the first place.

let's name it ns3.torproject.org, since that name is actually available (both ns2 and ns3 point to hetzner-hel1-02).

and let's retire fallax:

1. announcement
2. nagios
3. retire the host in fabric
4. remove from LDAP with ldapvi
5. power-grep
6. remove from tor-passwords
7. remove from DNSwl
8. remove from docs
9. remove from racks
10. remove from reverse DNS