**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.m_i2p_accept_incoming = args.GetBoolArg("-i2pacceptincoming", DEFAULT_I2P_ACCEPT_INCOMING);
    if (!node.connman->Start(*node.scheduler, connOptions)) {
        return false;
    }
     ********************************************************* Step 13: finished
     At this point, the RPC is "started", but still in warmup, which means it
     cannot yet be called. Before we make it callable, we need to make sure
     that the RPC's view of the best block is valid and consistent with
     ChainstateManager's active tip.
    
     If we do not do this, RPC's view of the best block will be height=0 and
     hash=0x0. This will lead to erroroneous responses for things like
     waitforblockheight.
    RPCNotifyBlockChange(WITH_LOCK(chainman.GetMutex(), return chainman.ActiveTip()));
    SetRPCWarmupFinished();
    uiInterface.InitMessage(_("Done loading").translated);
    for (const auto& client : node.chain_clients) {
        client->start(*node.scheduler);
    }
    BanMan* banman = node.banman.get();
    node.scheduler->scheduleEvery([banman]{
        banman->DumpBanlist();
    }, DUMP_BANS_INTERVAL);
    if (node.peerman) node.peerman->StartScheduledTasks(*node.scheduler);
#if HAVE_SYSTEM
    StartupNotify(args);
#endif
    return true;
}
bool StartIndexBackgroundSync(NodeContext& node)