(debt) Improve structure for APT repository setup

[penguineer]

In #33 we have introduced specific APT repository setup for Debian Trixie, made necessary due to #32. As this was a blocker for the application of this role to Debian Trixie systems and therefore system upgrades, we have left a number of questions unanswered:

• Can the deb822 format be applied more generally, i.e. what are legacy systems? (Debian 12 is minimum requirement and we still have Debian 11 in the mix for a while)
• Resolve the very specific tailoring to Debian Trixie
• How to structure the different setup variants while avoiding code duplication?
• Improve the way tasks for package installation are written