banlist.dat was introduced in #6310, storing nodes that were automatically banned (due to exceeding the misbehavior threshold) and manually banned (through the setban RPC).

Automatic bans were removed in #19219 and replaced with a discouragement filter. That filter is not saved to disk/persisted over shutdown/startup. The banlist.dat therefore now only contains addresses that have been banned through the setban rpc.

Since #15935, we have a specific file for configuration that is updated through the RPC and persisted over shutdown/startup, namely settings.json. We should therefore move the manual ban configuration from banlist.dat to settings.json. Doing so has a couple of benefits:

• settings.json is human readable and easily analyzable by any json parser.
• we wouldn't need to maintain custom [de]serialization code for the banlist.dat file.

banlist.dat is expected to be fairly small and infrequently updated, so disk space/performance are not huge concerns.