Rename the fallback input list to an "offer list"

In #24839 (moved), we want to replace the list of relay operators who have offered their relays as fallbacks, with signed statements in descriptors via a torrc option.

As part of that transition, we should rename the internal variables in the script, and the file in the repository. I suggest we use "fallback offer list".

We should also delete any remaining references to the blacklist, because it is obsolete, and the actual list was removed from the repository some time ago.

This change also has the benefit of avoiding the "white = good" metaphor, which can be confusing and problematic. For more details, see: https://tools.ietf.org/id/draft-knodel-terminology-00.html