Save the banlist in `banlist.json` instead of `banlist.dat`.
This makes it possible to store Tor v3 entries in the banlist on disk
(and any other addresses that cannot be serialized in addrv1 format).
Only read `banlist.dat` if it exists and `banlist.json` does not
exist (first start after an upgrade).
Supersedes https://github.com/bitcoin/bitcoin/pull/20904
Resolves https://github.com/bitcoin/bitcoin/issues/19748