`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.
{
    const CChainParams& chainparams = Params();
    auto connman = std::make_unique<CConnman>(0x1337, 0x1337, *m_node.addrman);
    auto peerLogic = PeerManager::make(chainparams, *connman, *m_node.addrman, nullptr,
                                       *m_node.scheduler, *m_node.chainman, *m_node.mempool, false);
    
    CAddress addr1(ip(0xa0b0c001), NODE_NONE);
    CNode dummyNode1(id++, ServiceFlags(NODE_NETWORK | NODE_WITNESS), INVALID_SOCKET, addr1,  0,  0, CAddress(),  "", ConnectionType::OUTBOUND_FULL_RELAY,  false);
    dummyNode1.SetCommonVersion(PROTOCOL_VERSION);
    peerLogic->InitializeNode(&dummyNode1);
    dummyNode1.fSuccessfullyConnected = true;
    
    {
        LOCK(cs_main);
        BOOST_CHECK(m_node.chainman->ActiveChain().Tip() != nullptr);
        BOOST_CHECK(m_node.chainman->ActiveChain().Tip()->nChainWork > 0);
    }
    
    {
        LOCK(dummyNode1.cs_sendProcessing);
        BOOST_CHECK(peerLogic->SendMessages(&dummyNode1)); 
    }
    {
        LOCK(dummyNode1.cs_vSend);
        BOOST_CHECK(dummyNode1.vSendMsg.size() > 0);
        dummyNode1.vSendMsg.clear();
    }
    int64_t nStartTime = GetTime();