Since avuitl 57.30.100 the pkt_duration is deprecated, and
the duration is to be used instead.
The units seems to match, and also from the avcodec decoder.c
the `frame->pkt_duration = frame->duration;` so does not seem
we need to do any conversion.
The change in FFmpeg is from 2022, which is a bit recent, so
the access to the duration is hidden behind a compatibility
API.