fix: apt-key deprecation warning/errors

[e-renna]

The use of apt-key has been deprecated for security reasons, and best practice is now to add keyrings to the directory /etc/apt/trusted.gpg.d/.
When adding the gpg key using the current method, the following error is being displayed:
Warning: apt-key is deprecated. Manage keyring files in trusted.gpg.d instead (see apt-key(8)).
Similar warning appears when running apt update:
Key is stored in legacy trusted.gpg keyring (/etc/apt/trusted.gpg), see the DEPRECATION section in apt-key(8) for details.
It should also be noted that apt-key(8) will last be available in Debian 11 and Ubuntu 22.04.

This PR edits one of the commands in the installation guide for Ubuntu-based distros, to use the new suggested method.

[CLAassistant]

All committers have signed the CLA.

[mlsmaycon]

Thanks, @e-renna appreciate your contribution.