BUG #17699: Outdated instructions for installing on Debian / Ubuntu systems

The following bug has been logged on the website:

Bug reference:      17699
Logged by:          Dirk Heinrichs
Email address:      dheinric@opentext.com
PostgreSQL version: 11.0
Operating system:   Debian / Ubuntu
Description:

Hi,

the installation instructions for Debian / Ubuntu describe a deprecated
method for fetching and storing the repository key. The "apt-key" utility
has been deprecated by Debian and Debian Bookworm (testing), as well as
Ubuntu 22.04 (current LTS) will emit annoying warnings about keys stored in
/etc/apt/trusted.gpg on each "apt-get update".

Please change instructions to follow
https://wiki.debian.org/DebianRepository/UseThirdParty and store the keyring
in "/usr/share/keyrings" (since it is also managed by the "pgdg-keyring"
package) for Debian versions >= Stretch (and their corresponding Ubuntu
versions).

This also affects the "pgdg-keyring" package, which stores the keyring in
"/usr/share/postgresql-common/pgdg" instead of "/usr/share/keyrings", so
this should also be fixed.

Thanks in advance...

    Dirk