**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/`
const std::optional<CService> addr{Lookup(onionArg, 9050, fNameLookup)};
            if (!addr.has_value() || !addr->IsValid()) {
                return InitError(strprintf(_("Invalid -onion address or hostname: '%s'"), onionArg));
            }
            onion_proxy = Proxy{addr.value(), proxyRandomize};
        }
    }
    if (onion_proxy.IsValid()) {
        SetProxy(NET_ONION, onion_proxy);
    } else {
         
         
        const bool listenonion_disabled{!args.GetBoolArg("-listenonion", DEFAULT_LISTEN_ONION)};
        if (onlynet_used_with_onion && listenonion_disabled) {
            return InitError(
                _("Outbound connections restricted to Tor (-onlynet=onion) but the proxy for "
                  "reaching the Tor network is not provided: none of -proxy, -onion or "
                  "-listenonion is given"));
        }
        SetReachable(NET_ONION, false);
    }
    for (const std::string& strAddr : args.GetArgs("-externalip")) {
        const std::optional<CService> addrLocal{Lookup(strAddr, GetListenPort(), fNameLookup)};
        if (addrLocal.has_value() && addrLocal->IsValid())
            AddLocal(addrLocal.value(), LOCAL_MANUAL);
        else
            return InitError(ResolveErrMsg("externalip", strAddr));
    }
#if ENABLE_ZMQ
    g_zmq_notification_interface = CZMQNotificationInterface::Create(