**TL;DR**: When upgrading from `Bitcoin Core 0.20.1` to `Bitcoin Core 0.21.0`, a downstream application using the HTTP JSON-RPC API started getting HTTP 403 "Forbidden".

Turns out it is because I have `rpcallowip=::/0` in `bitcoin.conf` which used to work but does not anymore.
As a workaround, `rpcallowip=0.0.0.0/0` works.

-----

I run bitcoind in docker container1, and some application using the HTTP JSON-RPC API of bitcoind is in docker container2. They are on the same "docker network".

In `bitcoin.conf` I have:
```
rpcauth=<...>
rpcbind=0.0.0.0:8332
rpcallowip=::/0
```
(bitcoind RPC is not actually exposed to the public internet; it is up to the docker layer and above to ensure this.)

The application in container2 is connecting through HTTP accessing an URL such as `http://user:pass@172.19.1.10:8332/`.

This setup used to work when running `Bitcoin Core 0.20.1` in container1.

It no longer works with `Bitcoin Core 0.21.0`.

-----

If I change my `bitcoin.conf` as below, it starts working again using 0.21:
```
rpcauth=<...>
rpcbind=0.0.0.0:8332
rpcallowip=0.0.0.0/0
```

So `rpcallowip=::/0` used to work but does not anymore; `rpcallowip=0.0.0.0/0` works.

-----

Easy way to test:
`curl --data-binary '{"jsonrpc":"1.0","id":"curltext","method":"getblockchaininfo","params":[]}' -H 'content-type:text/plain;' http://user:pass@172.19.1.10:8332/`
#if ENABLE_ZMQ
    if (g_zmq_notification_interface) {
        UnregisterValidationInterface(g_zmq_notification_interface.get());
        g_zmq_notification_interface.reset();
    }
#endif
    node.chain_clients.clear();
    UnregisterAllValidationInterfaces();
    GetMainSignals().UnregisterBackgroundSignalScheduler();
    node.kernel.reset();
    node.mempool.reset();
    node.fee_estimator.reset();
    node.chainman.reset();
    node.scheduler.reset();
    try {
        if (!fs::remove(GetPidFile(*node.args))) {
            LogPrintf("%s: Unable to remove PID file: File does not exist\n", __func__);
        }
    } catch (const fs::filesystem_error& e) {
        LogPrintf("%s: Unable to remove PID file: %s\n", __func__, fsbridge::get_filesystem_error_message(e));
    }
    LogPrintf("%s: done\n", __func__);
}
**
 * Signal handlers are very limited in what they are allowed to do.
 * The execution context the handler is invoked in is not guaranteed,
 * so we restrict handler operations to just touching variables:
 *
#ifndef WIN32
static void HandleSIGTERM(int)