rtpopusdepay, matroskamux: Fix invalid rate while muxing Opus in Matroska

• matroskamux: Set rate/channels in Opus tempalte caps

  For some reason these were missed, and if caps didn't have them, we would emit an invalid Matroska file with a 0 value for Sampling Frequency or channels.

• rtpopusdepay: Assume 48 kHz if sprop-maxcapturerate is missing

  This matches 7587, section 6.1:

  sprop-maxcapturerate: a hint about the maximum input sampling rate [...] bandwidths (Table 1). By default, the sender is assumed to have no limitations, i.e., 48000.