Use deb822 format for supported systems by default

[Programie]

Use Case

Since Ubuntu 25.10 only the new deb822 format (.sources instead of .list files) is supported. Unfortunately, the APT module does not automatically use this format which requires me to manually define source_format for EACH apt::source.

Describe the Solution You Would Like

Automatically use the correct value for source_format instead of always defaulting to "list".

As far as I know, Debian 11 and Ubuntu 24.04 are the first versions supporting the new deb822 format. Therefore, it would be useful to automatically use the new format on those systems by default.

Describe Alternatives You've Considered

I tried setting source_format as default using a Apt::Source {...} definition. But this only works in the current scope.

Using that before my node definitions works for any apt::source, independently where they are used (e.g. even for external modules). But this does not feel like a good long term solution.