Add Pop!_OS to the list of APT based distros

[Croydon]

Changelog: Feature: Add Pop!_OS to the list of APT based distributions.
Docs: Omit

There is no documentation about tools.os_info.with_apt as far as I can see?

https://pop.system76.com/

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Fixes conan-io/conan-center-index#1995

[jgsogo]

I've added the link in the description: https://pop.system76.com/