**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/`
g_genesis_wait_cv.wait_for(lock, std::chrono::milliseconds(500));
        }
        block_notify_genesis_wait_connection.disconnect();
    }
    if (ShutdownRequested()) {
        return false;
    }
     ********************************************************* Step 12: start node
     debug print
    {
        LOCK(cs_main);
        LogPrintf("block tree size = %u\n", chainman.BlockIndex().size());
        chain_active_height = chainman.ActiveChain().Height();
        if (tip_info) {
            tip_info->block_height = chain_active_height;
            tip_info->block_time = chainman.ActiveChain().Tip() ? chainman.ActiveChain().Tip()->GetBlockTime() : chainman.GetParams().GenesisBlock().GetBlockTime();
            tip_info->verification_progress = GuessVerificationProgress(chainman.GetParams().TxData(), chainman.ActiveChain().Tip());
        }
        if (tip_info && chainman.m_best_header) {
            tip_info->header_height = chainman.m_best_header->nHeight;
            tip_info->header_time = chainman.m_best_header->GetBlockTime();
        }
    }
    LogPrintf("nBestHeight = %d\n", chain_active_height);
    if (node.peerman) node.peerman->SetBestHeight(chain_active_height);
     Map ports with UPnP or NAT-PMP.
    StartMapPort(args.GetBoolArg("-upnp", DEFAULT_UPNP), args.GetBoolArg("-natpmp", DEFAULT_NATPMP));
    CConnman::Options connOptions;
    connOptions.nLocalServices = nLocalServices;
    connOptions.nMaxConnections = nMaxConnections;