[macOS/iOS] Use hardware sampling rates for audio I/O.

[bruvzg]

Instead of "mix_rate" from the project settings, use preferred OS values, since API can work only with the limited set of values depending on device.

Fixes https://github.com/godotengine/godot/issues/58180

TODO:

• I'm not sure if changes in the AudioStreamPlaybackMicrophone are sufficient to correctly resample it.
• We should track device changes and adjust sampling rates accordingly, but I'm not sure if Godot audio subsystem is capable of handling sampling rate changes.

Edit: I corrected a typo in "mix_rate". - @adamscott