Installation instructions for Debian/Ubuntu is wrong

[Mario Splivalo]

Hi, all.

Newer versions of Ubuntu/Debian strongly recommend using .soruces 
(deb822) over .list files for configuring package repositories.

Also, the documentation puts the pgadmin repository key file inside 
/usr/share/keyrings/ directory, which should not happen (the proper 
place for such files is /etc/apt/keyrings).

Where can I create a PR, or suggest a documentation change for recent 
versions of Ubuntu and Debian  to configure apt?

        Mario