`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_banned = banmap;
    m_is_dirty = true;
}
void BanMan::SweepBanned()
{
    int64_t now = GetTime();
    bool notify_ui = false;
    {
        LOCK(m_cs_banned);
        banmap_t::iterator it = m_banned.begin();
        while (it != m_banned.end()) {
            CSubNet sub_net = (*it).first;
            CBanEntry ban_entry = (*it).second;
            if (!sub_net.IsValid() || now > ban_entry.nBanUntil) {
                m_banned.erase(it++);
                m_is_dirty = true;
                notify_ui = true;
                LogPrint(BCLog::NET, "%s: Removed banned node ip/subnet from banlist.dat: %s\n", __func__, sub_net.ToString());
            } else
                ++it;
        }
    }
    // 
    if (notify_ui && m_client_interface) {
        m_client_interface->BannedListChanged();
    }
}
bool BanMan::BannedSetIsDirty()