v4l2src: Bump timestamp sync log message down to LOG(6) from INFO(4)

The following timestamp sync log message gets emitted each time the creation of a buffer succeeds:

0:18:42.989664641     1 0x5601c0efd770 INFO                 v4l2src gstv4l2src.c:957:gst_v4l2src_create:<v4l2src0> sync to 0:18:41.824194238 out ts 0:18:41.824194238

This appears to me to be in violation of the GStreamer log level guidelines, which state:

4 - INFO Logs all informational messages. These are typically used for events in the system that only happen once, or are important and rare enough to be logged at this level.

The result is that the logs at the INFO(4) level are saturated to the point of unusability for their intended purpose in application development, which is to track rare or singleton messages.

The one-token change in this merge request bumps the offending log statement from INFO(4) down to LOG(6), where its behavior is more in line with the guidelines:

6 - LOG Logs all log messages. These are messages for events that happen repeatedly during an object's lifetime; these include streaming and steady-state conditions.