[snap] upgrade to 4.2.8.2 and use metadata from appstream

Thanks for the heads up about the new version @rempt!

This PR bumps the version and extracts the metadata from AppStream instead of duplicating it in snapcraft.yaml.

This is now live in the snap store.

Note: I was hoping that the snap could use the kde-neon snapcraft extension by now since it drastically improves snap size, startup time and it makes building the snap a lot easier, but it seems at the moment only snaps published by KDE itself can use this extension at the moment, so I'll try to contact the KDE Neon devs to see if they want to open this for other applications using KDE frameworks. Long explanation to say "stay tuned, more good stuff should be coming in the future"