**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/`
nLocalServices = ServiceFlags(nLocalServices | NODE_BLOOM);
    if (args.GetIntArg("-rpcserialversion", DEFAULT_RPC_SERIALIZE_VERSION) < 0)
        return InitError(Untranslated("rpcserialversion must be non-negative."));
    if (args.GetIntArg("-rpcserialversion", DEFAULT_RPC_SERIALIZE_VERSION) > 1)
        return InitError(Untranslated("Unknown rpcserialversion requested."));
    
    {
        kernel::Notifications notifications{};
        ChainstateManager::Options chainman_opts_dummy{
            .chainparams = chainparams,
            .datadir = args.GetDataDirNet(),
            .notifications = notifications,
        };
        auto chainman_result{ApplyArgsManOptions(args, chainman_opts_dummy)};
        if (!chainman_result) {
            return InitError(util::ErrorString(chainman_result));
        }
        BlockManager::Options blockman_opts_dummy{
            .chainparams = chainman_opts_dummy.chainparams,
            .blocks_dir = args.GetBlocksDirPath(),
            .notifications = chainman_opts_dummy.notifications,
        };
        auto blockman_result{ApplyArgsManOptions(args, blockman_opts_dummy)};
        if (!blockman_result) {
            return InitError(util::ErrorString(blockman_result));
        }
    }
    return true;
}
static bool LockDataDirectory(bool probeOnly)