movenc: mark Opus encapsulation as stable

The specifications are de-facto frozen now as they've already been used in
production for years, the author has indicated reluctance on IRC to change
it further, and the only potential changes would, from what I understand,
be forward-compatible.

libavformat/movenc.c

@@ -6500,7 +6500,8 @@ static int mov_init(AVFormatContext *s)
                     av_log(s, AV_LOG_ERROR, "%s only supported in MP4.\n", avcodec_get_name(track->par->codec_id));
                     return AVERROR(EINVAL);
                 }
-                if (s->strict_std_compliance > FF_COMPLIANCE_EXPERIMENTAL) {
+                if (track->par->codec_id != AV_CODEC_ID_OPUS &&
+                    s->strict_std_compliance > FF_COMPLIANCE_EXPERIMENTAL) {
                     av_log(s, AV_LOG_ERROR,
                            "%s in MP4 support is experimental, add "
                            "'-strict %d' if you want to use it.\n",