**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/`
connOptions.vSeedNodes = args.GetArgs("-seednode");
     Initiate outbound connections unless connect=0
    connOptions.m_use_addrman_outgoing = !args.IsArgSet("-connect");
    if (!connOptions.m_use_addrman_outgoing) {
        const auto connect = args.GetArgs("-connect");
        if (connect.size() != 1 || connect[0] != "0") {
            connOptions.m_specified_outgoing = connect;
        }
        if (!connOptions.m_specified_outgoing.empty() && !connOptions.vSeedNodes.empty()) {
            LogPrintf("-seednode is ignored when -connect is used\n");
        }
        if (args.IsArgSet("-dnsseed") && args.GetBoolArg("-dnsseed", DEFAULT_DNSSEED) && args.IsArgSet("-proxy")) {
            LogPrintf("-dnsseed is ignored when -connect is used and -proxy is specified\n");
        }
    }
    const std::string& i2psam_arg = args.GetArg("-i2psam", "");
    if (!i2psam_arg.empty()) {
        const std::optional<CService> addr{Lookup(i2psam_arg, 7656, fNameLookup)};
        if (!addr.has_value() || !addr->IsValid()) {
            return InitError(strprintf(_("Invalid -i2psam address or hostname: '%s'"), i2psam_arg));
        }
        SetProxy(NET_I2P, Proxy{addr.value()});
    } else {
        if (args.IsArgSet("-onlynet") && IsReachable(NET_I2P)) {
            return InitError(
                _("Outbound connections restricted to i2p (-onlynet=i2p) but "
                  "-i2psam is not provided"));
        }
        SetReachable(NET_I2P, false);
    }