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.

Signed-off-by: Hugues Fruchet <hugues.fruchet@foss.st.com>