(MODULES-9695) Debian: use modern APT keyring format

[kenyon]

This updates puppet_agent::osfamily::debian to use modern APT keyrings (https://github.com/puppetlabs/puppetlabs-apt/pull/1128) instead of the deprecated apt-key method used by apt::key and apt::source.key without name.

For this to work properly, it needs a release of puppetlabs-apt which contains https://github.com/puppetlabs/puppetlabs-apt/pull/1128, which should be the release after v9.1.0.

This also removes the legacy key, because keys not used for signing package sources aren't needed.

/etc/pki is not needed anymore (also this directory is a RedHatism) because keyrings are now stored in the default location of /etc/apt/keyrings. We don't clean it up though, in case people are using the files there for something else.