Updates apt::source usage

[amateo]

The use of key_source in apt::source is deprecated since puppetlabs/apt version 2.1.1 (at least) and removed in version 4.0.0.

[noqqe]

Please merge, unless this is in the codebase, there is no way installing the module with librarian-puppet.

apt dependency is way too old.

[jarro2783]

+1