multidupsink: Use gst_net_utils_set_socket_dscp for QoS DSCP

Util function in net library exists for setting QoS DSCP on socket, hence
use it to simplify code.
2 jobs from qos_dscp in 2 minutes and 46 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Manifest
passed #43665
manifest

00:00:21

 
  Build
passed #43666
build fedora x86_64

00:02:25