**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/`
#include <nodevalidation_cache_args.h>
#include <policyfeerate.h>
#include <policyfees.h>
#include <policyfees_args.h>
#include <policypolicy.h>
#include <policysettings.h>
#include <protocol.h>
#include <rpcblockchain.h>
#include <rpcregister.h>
#include <rpcserver.h>
#include <rpcutil.h>
#include <scheduler.h>
#include <scriptsigcache.h>
#include <shutdown.h>
#include <sync.h>
#include <timedata.h>
#include <torcontrol.h>
#include <txdb.h>
#include <txmempool.h>
#include <utilasmap.h>
#include <utilchaintype.h>
#include <utilcheck.h>
#include <utilfs.h>
#include <utilfs_helpers.h>
#include <utilmoneystr.h>
#include <utilresult.h>
#include <utilstrencodings.h>
#include <utilstring.h>
#include <utilsyserror.h>
#include <utilthread.h>