Adding a line to Changelog about Opus Matroska Support

Next release of ffmpeg will support embedding Opus in Matroska
container. Adding that to the Changelog.

Signed-Off By: Vignesh Venkatasubramanian <vigneshv@google.com>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
lavf/matroska: Removing experimental suffix for Opus

Removing the /EXPERIMENTAL suffix for codec id field for Opus. Doing this
as the implementation of embedding Opus in Matroska is complete.

Signed-Off By: Vignesh Venkatasubramanian <vigneshv@google.com>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>