Move supported systems/platforms into SUPPORTED_PLATFORMS

What does this Merge Request do and why?

This MR performs the following:

• Adds support for PopOS to install packages from packages.txt
• Refactor the logic that looks at supported platforms to solely use SUPPORTED_PLATFORMS

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

Closes #1214 (closed)