package/opus: bump to version 1.3

package/opus: bump to version 1.3

This Opus 1.3 major release again brings quality improvements, new
features, and bug fixes. Changes since 1.2.x include:

- Improvements to the VAD and speech/music classification using an RNN
- Support for ambisonics coding using channel mapping families 2 and 3
- Improvements to stereo speech coding at low bitrate
- Using wideband encoding down to 9 kb/s
- Making it possible to use SILK down to bitrates around 5 kb/s
- Minor quality improvement on tones
- Enabling the spec fixes in RFC 8251 by default
- Security/hardening improvements

Notable bug fixes include:

- Fixes to the CELT PLC
- Bandwidth detection fixes

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>