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.

tools/mtmd/mtmd.cpp

@@ -912,7 +912,7 @@ bool mtmd_support_audio(mtmd_context * ctx) {
     return ctx->ctx_a != nullptr;
 }
 
-int mtmd_get_audio_bitrate(mtmd_context * ctx) {
+int mtmd_get_audio_sample_rate(mtmd_context * ctx) {
     if (!ctx->ctx_a) {
         return -1;
     }