Support for mono-channel microphones

### Steps to reproduce

1. Have a mono microphone, either purchase one, or use an audio routing modifier program such as [SoundFlower (or Loopback)](https://rogueamoeba.com/freebies/soundflower/) on MacOS, or qjackctl/qpwgraph/helvum on Linux (supports Pipewire or JACK).
2. Join an Element call instance with someone.
3. Note that the other participant can only hear you in one channel.

It's nice that we support stereo audio, unlike other applications like Jitsi which just combine the two channels before sending it out. But this is detrimental with mono mics as:

* You come through in one ear only.
* You're (potentially?) sending data for two channels of audio while only one has anything actually useful.

### Outcome

#### What did you expect?

I'd expect to be able to configure my microphone with a "Mono" checkbox in audio settings.

#### What happened instead?

The only workaround I have at the moment is to switch to Jitsi, or use one of the above mentioned programs to duplicate the L channel into the R channel so that people don't only hear me in one ear.

### Operating system

Arch Linux

### Browser information

Firefox 103.0.1

### URL for webapp

call.element.io

### Will you send logs?

No