audio_core: Convert LOG_CRITICAL + UNREACHABLE over to UNIMPLEMENTED/UNIMPLEMENTED_MSG

These two macros being used in tandem were used prior to the
introduction of UNIMPLEMENTED and UNIMPLEMENTED_MSG. This provides
equivalent behavior, just with less typing/reading involved.

src/audio_core/audio_out.cpp

@@ -22,8 +22,7 @@ static Stream::Format ChannelsToStreamFormat(u32 num_channels) {
         return Stream::Format::Multi51Channel16;
     }
 
-    LOG_CRITICAL(Audio, "Unimplemented num_channels={}", num_channels);
-    UNREACHABLE();
+    UNIMPLEMENTED_MSG("Unimplemented num_channels={}", num_channels);
     return {};
 }