mtmd : rename mtmd_get_audio_bitrate to mtmd_get_audio_sample_rate (#20105)

This commit renames the the function `mtmd_get_audio_bitrate` to
`mtmd_get_audio_sample_rate` to better reflect its purpose.

The motivation for this is that the function currently returns the audio
sample rate, not the bitrate (sample_rate × bit_depth × channels), and
that is how it is used in the code as well.

This is a breaking change, but I believe mtmd is still in
experimental/development phase so it might be alright to simply rename.