`banlist.dat` was introduced in https://github.com/bitcoin/bitcoin/pull/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 https://github.com/bitcoin/bitcoin/pull/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 https://github.com/bitcoin/bitcoin/pull/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.
else
            return InitError(ResolveErrMsg("externalip", strAddr));
    }
    
    if (args.IsArgSet("-asmap")) {
        fs::path asmap_path = fs::path(args.GetArg("-asmap", ""));
        if (asmap_path.empty()) {
            asmap_path = DEFAULT_ASMAP_FILENAME;
        }
        if (!asmap_path.is_absolute()) {
            asmap_path = gArgs.GetDataDirNet() / asmap_path;
        }
        if (!fs::exists(asmap_path)) {
            InitError(strprintf(_("Could not find asmap file %s"), asmap_path));
            return false;
        }
        std::vector<bool> asmap = CAddrMan::DecodeAsmap(asmap_path);
        if (asmap.size() == 0) {
            InitError(strprintf(_("Could not parse asmap file %s"), asmap_path));
            return false;
        }
        const uint256 asmap_version = SerializeHash(asmap);
        node.connman->SetAsmap(std::move(asmap));
        LogPrintf("Using asmap version %s for IP bucketing\n", asmap_version.ToString());
    } else {
        LogPrintf("Using /16 prefix for IP bucketing\n");
    }
#if ENABLE_ZMQ
    g_zmq_notification_interface = CZMQNotificationInterface::Create();
    if (g_zmq_notification_interface) {