Stage.scale has been moved to Game.scale. The same game scaling properties exist as before, but now accessed via Game.scale instead.
Stage.aspectRatio has been moved to StageScaleMode.sourceAspectRatio (so now game.scale.sourceAspectRatio)
Stage.scaleMode has been moved to StageScaleMode.scaleMode (so now game.scale.scaleMode)
Stage.fullScreenScaleMode has been moved to StageScaleMode.fullScreenScaleMode (so now game.scale.fullScreenScaleMode)
Stage.canvas has been removed. It was only ever an alias for Game.canvas anyway, so access it via that instead.