`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.
#include <banman.h>
#include <netaddress.h>
#include <node/ui_interface.h>
#include <util/system.h>
#include <util/time.h>
#include <util/translation.h>
BanMan::BanMan(fs::path ban_file, CClientUIInterface* client_interface, int64_t default_ban_time)
    : m_client_interface(client_interface), m_ban_db(std::move(ban_file)), m_default_ban_time(default_ban_time)
{
    if (m_client_interface) m_client_interface->InitMessage(_("Loading banlist…").translated);
    int64_t n_start = GetTimeMillis();
    m_is_dirty = false;
    banmap_t banmap;
    if (m_ban_db.Read(banmap)) {
        SetBanned(banmap);        // 
        SetBannedSetDirty(false); // 
        SweepBanned();            // 
        LogPrint(BCLog::NET, "Loaded %d banned node ips/subnets from banlist.dat  %dms\n",
            m_banned.size(), GetTimeMillis() - n_start);
    } else {
        LogPrintf("Recreating banlist.dat\n");
        SetBannedSetDirty(true); // 
        DumpBanlist();
    }
}
BanMan::~BanMan()