`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.
{
    LOCK(m_cs_banned);
    m_discouraged.insert(net_addr.GetAddrBytes());
}
void BanMan::Ban(const CSubNet& sub_net, int64_t ban_time_offset, bool since_unix_epoch)
{
    CBanEntry ban_entry(GetTime());
    int64_t normalized_ban_time_offset = ban_time_offset;
    bool normalized_since_unix_epoch = since_unix_epoch;
    if (ban_time_offset <= 0) {
        normalized_ban_time_offset = m_default_ban_time;
        normalized_since_unix_epoch = false;
    }
    ban_entry.nBanUntil = (normalized_since_unix_epoch ? 0 : GetTime()) + normalized_ban_time_offset;
    {
        LOCK(m_cs_banned);
        if (m_banned[sub_net].nBanUntil < ban_entry.nBanUntil) {
            m_banned[sub_net] = ban_entry;
            m_is_dirty = true;
        } else
            return;
    }
    if (m_client_interface) m_client_interface->BannedListChanged();
    //
    DumpBanlist();
}
bool BanMan::Unban(const CNetAddr& net_addr)