**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/`
InitWarning(BadPortWarning("-bind", bind_addr.value().GetPort()));
                }
                continue;
            }
        } else {
            const std::string network_type = bind_arg.substr(index + 1);
            if (network_type == "onion") {
                const std::string truncated_bind_arg = bind_arg.substr(0, index);
                bind_addr = Lookup(truncated_bind_arg, BaseParams().OnionServiceTargetPort(), false);
                if (bind_addr.has_value()) {
                    connOptions.onion_binds.push_back(bind_addr.value());
                    continue;
                }
            }
        }
        return InitError(ResolveErrMsg("bind", bind_arg));
    }
    for (const std::string& strBind : args.GetArgs("-whitebind")) {
        NetWhitebindPermissions whitebind;
        bilingual_str error;
        if (!NetWhitebindPermissions::TryParse(strBind, whitebind, error)) return InitError(error);
        connOptions.vWhiteBinds.push_back(whitebind);
    }
     If the user did not specify -bind= or -whitebind= then we bind
     on any a
    connOptions.bind_on_any = args.GetArgs("-bind").empty() && args.GetArgs("-whitebind").empty();
     Emit a warning if a bad port is given to -port= but only if -bind and -whitebind are not
     given, because if they are, then -port= is ignored.
    if (connOptions.bind_on_any && args.IsArgSet("-port")) {
        const uint16_t port_arg = args.GetIntArg("-port", 0);