Fix ffmpeg channel_layout obsolete warnings

[c72578]

Use ch_layout instead of the obsolete channel_layout and ch_layout.nb_channels instead of obsolete channels, which fixes the following warnings:

• Warning CS0618 'AVCodecParameters.channel_layout' is obsolete: 'use ch_layout'
• Warning CS0618 'AVCodecParameters.channels' is obsolete: 'use ch_layout.nb_channels'
• Warning CS0618 'AVCodecContext.channels' is obsolete: 'use ch_layout.nb_channels'