`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.
{
    os << num.ToString();
    return os;
}
BasicTestingSetup::BasicTestingSetup(const std::string& chainName, const std::vector<const char*>& extra_args)
    : m_path_root{fs::temp_directory_path() / "test_common_" PACKAGE_NAME / g_insecure_rand_ctx_temp_path.rand256().ToString()},
      m_args{}
{
    m_node.args = &gArgs;
    const std::vector<const char*> arguments = Cat(
        {
            "dummy",
            "-printtoconsole=0",
            "-logsourcelocations",
            "-logtimemicros",
            "-logthreadnames",
            "-debug",
            "-debugexclude=libevent",
            "-debugexclude=leveldb",
        },
        extra_args);
    util::ThreadRename("test");
    fs::create_directories(m_path_root);
    m_args.ForceSetArg("-datadir", m_path_root.string());
    gArgs.ForceSetArg("-datadir", m_path_root.string());