Fix apt-key deprecated message

[miluxhd]

Pull Request (PR) description

In the current implementation, apt::source stores GPG keys in the trusted.gpg file, which is no longer recommended. Additionally, with the deprecation of apt-key, running the apt update command triggers warnings on Debian-based distributions.
This PR modifies the handling of GPG keys so that they are stored in the keyrings directory, aligning with best practices and avoiding the deprecation warnings associated with apt-key

[kenyon]

Can you also update the README to say that at least puppetlabs-apt v9.2.0 is needed? That's the version which added support for "modern keyrings". Here in the readme:

puppet-nginx/README.md

Lines 19 to 21 in 12d6b62

	* apt is now a soft dependency. If your system uses apt, you'll need to
	configure an appropriate version of the apt module. Version 4.4.0 or higher is
	recommended because of the proper handling of `apt-transport-https`.

https://github.com/puppetlabs/puppetlabs-apt/blob/main/CHANGELOG.md#v920---2023-12-04

[TheMeier]

given the "new" version for the dependency, should this be marked backward-incompatible?

[kenyon]

given the "new" version for the dependency, should this be marked backward-incompatible?

I think yes, because users of this module who use it with puppetlabs-apt would need to update their puppetlabs-apt module. Same as how we've done for this kind of change in other modules.

[kenyon]

Oops, missed one other thing. The filenames need to end in .asc.

I think this also indicates that we are lacking acceptance test coverage.