Add support for Pop!_OS in package_manager

[jwillikers]

Fixes #15930.

Changelog: Fix: Add Pop!_OS to the distros using apt-get as system package manager.
Docs: Omit

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[Croydon]

Weird that it got lost. I have added it once #7237

[memsharded]

Oh, the SystemPackageManager got a totally new rewrite from scratch some time ago and #7237 logic was moved to some dicts elsewhere in package_manager.py, so probably it was a miss in the rewrite.