Use modern APT keyrings on Debian family

This makes use of puppetlabs/puppetlabs-apt#1128 to store the public key
in `/etc/apt/keyrings` and add a `signed-by` option to the
`sources.list.d` entry.

Author: smortex

manifests/repo.pp

@@ -29,8 +29,7 @@
           repos    => "${fact('os.name')}-${fact('os.distro.codename')}".downcase,
           release  => 'stable',
           key      => {
-            ensure => 'refreshed',
-            id     => '2E6994033390DE82D8E6A2D16E187A4C6694369F',
+            name   => 'syslog-ng',
             source => "${release_url}syslog-ng-ose-pub.asc",
           },
           include  => {