rtpopusdepay: Accept MULTIOPUS caps
gst_sdp_media_get_caps_from_media() always converts SDP encoding names to upper case, so add support for upper-case 'MULTIOPUS' to the depayloader sink caps to fix transporting multichannel opus through webrtc
gst_sdp_media_get_caps_from_media() always converts SDP encoding names to upper case, so add support for upper-case 'MULTIOPUS' to the depayloader sink caps to fix transporting multichannel opus through webrtc