BUG #16995: Need repository key to access old distributions from https://apt-archive.postgresql.org/

The following bug has been logged on the website:

Bug reference:      16995
Logged by:          Marc Moy
Email address:      moymarc453@gmail.com
PostgreSQL version: 9.6.21
Operating system:   Ubuntu
Description:

We are running into errors when installing Postgres 9.6 on Ubuntu. We
noticed the distributions hosted on  `http://apt.postgresql.org` was
returning 404 so we changed our installation scripts to refer to
`https://apt-archive.postgresql.org/` instead as described here:
https://www.postgresql.org/message-id/YBMtd6nRuXyU2zS4%40msg.df7cb.de

However, we are now running into the following error now:
```
16:40:34 WARNING: The following packages cannot be authenticated!
16:40:34   libpq5 libpq-dev pgdg-keyring postgresql-client-common
postgresql-client-9.6
16:40:34   postgresql-common postgresql-9.6 postgresql-contrib-9.6
```

We believe its because the repository key we are using does not apply
anymore for the new `apt-archive` host. For reference, here is the
repository key we are using:
http://apt.postgresql.org/pub/repos/apt/ACCC4CF8.asc

Could someone please advise what the new repository key is for
https://apt-archive.postgresql.org/?

Thanks