Ubuntu and Debian Download/Installation instructions enhancement

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/16/bug-reporting.html
Description:

Dear PostgreSQL Documentation Team,

I hope this message finds you well. As a user and admirer of PostgreSQL, I
greatly appreciate the effort and detail that goes into maintaining the
project documentation and would like to take this opportunity to thank you
for curating such a well crafted, valuable resource.

This week I installed PostgreSQL 16 into an Ubuntu 22.04 LXD container, per
these instructions https://www.postgresql.org/download/linux/ubuntu/ and
observed a deprecation warning for apt-key on the following command:

wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | sudo
apt-key add -

Warning: apt-key is deprecated. Manage keyring files in trusted.gpg.d
instead (see apt-key(8)).


I subsequently referred to the documentation which notes:

DEPRECATION
       Except for using apt-key del in maintainer scripts, the use of
apt-key is deprecated. This section shows how to replace
       existing use of apt-key.

       If your existing use of apt-key add looks like this:

       wget -qO- https://myrepo.example/myrepo.asc | sudo apt-key add -

       Then you can directly replace this with (though note the
recommendation below):

       wget -qO- https://myrepo.example/myrepo.asc | sudo tee
/etc/apt/trusted.gpg.d/myrepo.asc

       Make sure to use the "asc" extension for ASCII armored keys and the
"gpg" extension for the binary OpenPGP format (also
       known as "GPG key public ring"). The binary OpenPGP format works for
all apt versions, while the ASCII armored format works
       for apt version >= 1.4.

       Recommended: Instead of placing keys into the /etc/apt/trusted.gpg.d
directory, you can place them anywhere on your
       filesystem by using the Signed-By option in your sources.list and
pointing to the filename of the key. See sources.list(5)
       for details. Since APT 2.4, /etc/apt/keyrings is provided as the
recommended location for keys not managed by packages.
       When using a deb822-style sources.list, and with apt version >= 2.4,
the Signed-By option can also be used to include the
       full ASCII armored keyring directly in the sources.list without an
additional file.


This deprecation notice was introduced in Ubuntu Groovy Gorilla (20.10)
which was released in October 2020. I therefore propose a documentation
change for the Ubuntu instructions, altering  the line command from:

wget --quiet -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | sudo
apt-key add -

to:

wget -qO- https://www.postgresql.org/media/keys/ACCC4CF8.asc | sudo tee
/etc/apt/trusted.gpg.d/postgresql.asc

as per the apt-key manual recommendation. This approach has been successful
in my testing and has yielded no warnings or errors. The documentation
proposes that an alternative location could be specified for the key, of
which I do not have an opinion of at this time, therefore I would appreciate
input on this location.

Since Ubuntu tracks the upstream Debian repository, I propose the Debian
instructions (https://www.postgresql.org/download/linux/debian/) are also
updated with this change.

Whilst this is not yet strictly a bug it will become one if/ when the
deprecated apt-key functionality is removed.  Resolving the warning would
also improve the experience for new/ inexperienced users who are potentially
installing PostgreSQL for the first time and do not understand the message.
I therefore think it prudent to review and update these instructions at the
earliest convenience. 

Thank you very much for considering my request. I look forward to your
guidance on how best to proceed with this enhancement.

Best regards,

Daniel Dewberry