Update fallback script to match Tor bootstrap changes
In #24661 (moved), we make clients bootstrap from reasonably old consensuses (expired in the last 24 hours).
In #28591 (moved), we make clients accept future consensuses as long as they're reasonably future consensuses (valid up to 24 hours in the future), and relays serve those consensuses.
We need to change the fallback checks to match these changes in Tor.