Deprecated usage of apt-key.

[joshwlewis]

apt-get is throwing this error:

W: http://apt.postgresql.org/pub/repos/apt/dists/mantic-pgdg/InRelease: Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.

We're adding keys for pgdg in the wrong way.

Potential solutions:

• https://wiki.postgresql.org/wiki/Apt#Manual_Repository_Configuration
• https://askubuntu.com/a/1307181

GUS-W-15213125.