**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/`
argsman.AddArg("-natpmp", strprintf("Use NAT-PMP to map the listening port (default: %s)", DEFAULT_NATPMP ? "1 when listening and no -proxy" : "0"), ArgsManager::ALLOW_ANY, OptionsCategory::CONNECTION);
#else
    hidden_args.emplace_back("-natpmp");
#endif 
    argsman.AddArg("-whitebind=<[permissions@]addr>", "Bind to the given address and add permission flags to the peers connecting to it. "
        "Use [host]:port notation for IPv6. Allowed permissions: " + Join(NET_PERMISSIONS_DOC, ", ") + ". "
        "Specify multiple permissions separated by commas (default: download,noban,mempool,relay). Can be specified multiple times.", ArgsManager::ALLOW_ANY, OptionsCategory::CONNECTION);
    argsman.AddArg("-whitelist=<[permissions@]IP address or network>", "Add permission flags to the peers connecting from the given IP address (e.g. 1.2.3.4) or "
        "CIDR-notated network (e.g. 1.2.3.024). Uses the same permissions as "
        "-whitebind. Can be specified multiple times." , ArgsManager::ALLOW_ANY, OptionsCategory::CONNECTION);
    g_wallet_init_interface.AddWalletOptions(argsman);
#if ENABLE_ZMQ
    argsman.AddArg("-zmqpubhashblock=<address>", "Enable publish hash block in <address>", ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
    argsman.AddArg("-zmqpubhashtx=<address>", "Enable publish hash transaction in <address>", ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
    argsman.AddArg("-zmqpubrawblock=<address>", "Enable publish raw block in <address>", ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
    argsman.AddArg("-zmqpubrawtx=<address>", "Enable publish raw transaction in <address>", ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
    argsman.AddArg("-zmqpubsequence=<address>", "Enable publish hash block and tx sequence in <address>", ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
    argsman.AddArg("-zmqpubhashblockhwm=<n>", strprintf("Set publish hash block outbound message high water mark (default: %d)", CZMQAbstractNotifier::DEFAULT_ZMQ_SNDHWM), ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
    argsman.AddArg("-zmqpubhashtxhwm=<n>", strprintf("Set publish hash transaction outbound message high water mark (default: %d)", CZMQAbstractNotifier::DEFAULT_ZMQ_SNDHWM), ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
    argsman.AddArg("-zmqpubrawblockhwm=<n>", strprintf("Set publish raw block outbound message high water mark (default: %d)", CZMQAbstractNotifier::DEFAULT_ZMQ_SNDHWM), ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
    argsman.AddArg("-zmqpubrawtxhwm=<n>", strprintf("Set publish raw transaction outbound message high water mark (default: %d)", CZMQAbstractNotifier::DEFAULT_ZMQ_SNDHWM), ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
    argsman.AddArg("-zmqpubsequencehwm=<n>", strprintf("Set publish hash sequence message high water mark (default: %d)", CZMQAbstractNotifier::DEFAULT_ZMQ_SNDHWM), ArgsManager::ALLOW_ANY, OptionsCategory::ZMQ);
#else
    hidden_args.emplace_back("-zmqpubhashblock=<address>");
    hidden_args.emplace_back("-zmqpubhashtx=<address>");
    hidden_args.emplace_back("-zmqpubrawblock=<address>");
    hidden_args.emplace_back("-zmqpubrawtx=<address>");
    hidden_args.emplace_back("-zmqpubsequence=<n>");
    hidden_args.emplace_back("-zmqpubhashblockhwm=<n>");
    hidden_args.emplace_back("-zmqpubhashtxhwm=<n>");