gst-decoder.c: switch to decodebin3

decodebin3 is no more experimental since GStreamer-1.22: https://gstreamer.freedesktop.org/releases/1.22 decodebin3 is now prefered to decodebin till GStreamer-1.24: https://gstreamer.freedesktop.org/releases/1.24

This solves negotiation issues encountered with V4L2 stateless hardware video decoders when using kmscube video playback option.