**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(strprintf(_(
                    "Disk space for %s may not accommodate the block files. " \
                    "Approximately %u GB of data will be stored in this directory."
                ),
                fs::quoted(fs::PathToString(args.GetBlocksDirPath())),
                chainparams.AssumedBlockchainSize()
            ));
        }
    }
     Either install a handler to notify us when genesis activates, or set fHaveGenesis directly.
     No locking, as this happens before any background thread is started.
    boost::signals2::connection block_notify_genesis_wait_connection;
    if (WITH_LOCK(chainman.GetMutex(), return chainman.ActiveChain().Tip() == nullptr)) {
        block_notify_genesis_wait_connection = uiInterface.NotifyBlockTip_connect(std::bind(BlockNotifyGenesisWait, std::placeholders::_2));
    } else {
        fHaveGenesis = true;
    }
#if HAVE_SYSTEM
    const std::string block_notify = args.GetArg("-blocknotify", "");
    if (!block_notify.empty()) {
        uiInterface.NotifyBlockTip_connect([block_notify](SynchronizationState sync_state, const CBlockIndex* pBlockIndex) {
            if (sync_state != SynchronizationState::POST_INIT || !pBlockIndex) return;
            std::string command = block_notify;
            ReplaceAll(command, "%s", pBlockIndex->GetBlockHash().GetHex());
            std::thread t(runCommand, command);
            t.detach(); 
        });
    }
#endif
    std::vector<fs::path> vImportFiles;