Add check for major OS version updates

[m-roberts]

As well as checking for package updates, major version updates of pi-topOS should be tracked.
Perhaps we can do point-releases similar to Debian (e.g. 10.4) where new major version means new underlying OS. This would allow web portal to know if a new version is out, which we can use to notify users that they need to update.
This could be handled by a package in https://github.com/pi-top/pt-os-core/ somehow - possibly related to news updates regarding pi-topOS that web portal can show.