`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(nodes[0]->cs_sendProcessing);
        BOOST_CHECK(peerLogic->SendMessages(nodes[0]));
    }
    BOOST_CHECK(banman->IsDiscouraged(addr[0]));
    BOOST_CHECK(nodes[0]->fDisconnect);
    BOOST_CHECK(!banman->IsDiscouraged(other_addr)); 
    nodes[1] = new CNode{id++, NODE_NETWORK, INVALID_SOCKET, addr[1],  1,
                          1, CAddress(),  "",
                         ConnectionType::INBOUND,  false};
    nodes[1]->SetCommonVersion(PROTOCOL_VERSION);
    peerLogic->InitializeNode(nodes[1]);
    nodes[1]->fSuccessfullyConnected = true;
    connman->AddTestNode(*nodes[1]);
    peerLogic->Misbehaving(nodes[1]->GetId(), DISCOURAGEMENT_THRESHOLD - 1,  "");
    {
        LOCK(nodes[1]->cs_sendProcessing);
        BOOST_CHECK(peerLogic->SendMessages(nodes[1]));
    }
    
    BOOST_CHECK(banman->IsDiscouraged(addr[0]));
    BOOST_CHECK(nodes[0]->fDisconnect);
    
    BOOST_CHECK(!banman->IsDiscouraged(addr[1]));
    BOOST_CHECK(!nodes[1]->fDisconnect);
    peerLogic->Misbehaving(nodes[1]->GetId(), 1,  ""); 
    {
        LOCK(nodes[1]->cs_sendProcessing);
        BOOST_CHECK(peerLogic->SendMessages(nodes[1]));
    }