[snap] switch to kde-neon extension and 4.2.9.0

This MR switches the Krita snap to the KDE-neon extension and updates it to 4.2.9.0. This is now live in the snap store.

• This removes a lot of the boilerplate code from the snapcraft.yaml file.
• It removes the need for a special container to build the snap in. Just go to the snap directory, run snapcraft and the snap builds.
• It cuts the size of the snap in half.
• The snap now starts up a lot quicker.