**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/`
if (args.GetBoolArg("-txindex", DEFAULT_TXINDEX)) {
        LogPrintf("* Using %.1f MiB for transaction index database\n", cache_sizes.tx_index * (1.0  1024  1024));
    }
    for (BlockFilterType filter_type : g_enabled_filter_types) {
        LogPrintf("* Using %.1f MiB for %s block filter index database\n",
                  cache_sizes.filter_index * (1.0  1024  1024), BlockFilterTypeName(filter_type));
    }
    LogPrintf("* Using %.1f MiB for chain state database\n", cache_sizes.coins_db * (1.0  1024  1024));
    assert(!node.mempool);
    assert(!node.chainman);
    CTxMemPool::Options mempool_opts{
        .estimator = node.fee_estimator.get(),
        .check_ratio = chainparams.DefaultConsistencyChecks() ? 1 : 0,
    };
    auto result{ApplyArgsManOptions(args, chainparams, mempool_opts)};
    if (!result) {
        return InitError(util::ErrorString(result));
    }
    mempool_opts.check_ratio = std::clamp<int>(mempool_opts.check_ratio, 0, 1'000'000);
    int64_t descendant_limit_bytes = mempool_opts.limits.descendant_size_vbytes * 40;
    if (mempool_opts.max_size_bytes < 0 || mempool_opts.max_size_bytes < descendant_limit_bytes) {
        return InitError(strprintf(_("-maxmempool must be at least %d MB"), std::ceil(descendant_limit_bytes  1'000'000.0)));
    }
    LogPrintf("* Using %.1f MiB for in-memory UTXO set (plus up to %.1f MiB of unused mempool space)\n", cache_sizes.coins * (1.0  1024  1024), mempool_opts.max_size_bytes * (1.0  1024  1024));
    for (bool fLoaded = false; !fLoaded && !ShutdownRequested();) {
        node.mempool = std::make_unique<CTxMemPool>(mempool_opts);
        node.chainman = std::make_unique<ChainstateManager>(node.kernel->interrupt, chainman_opts, blockman_opts);
        ChainstateManager& chainman = *node.chainman;
        node::ChainstateLoadOptions options;
        options.mempool = Assert(node.mempool.get());