We have multiple properties referring to "rates", some of which go by a
generic "rate" name without specifying whether it is a frame rate or a
sample rate.

This change deprecates:

- `AudioCodecContext.rate` (use `AudioCodecContext.sample_rate`)
- `AudioFrame.rate` (use `AudioFrame.sample_rate`)
- `VideoCodecContext.rate` (use `VideoCodecContext.framerate`)