[webrtc-pc] Mark voiceActivityFlag as at risk (#2496)

dontcallmedom has just created a new issue for https://github.com/w3c/webrtc-pc:

== Mark voiceActivityFlag as at risk ==
The spec requires that`RTCRtpSynchronizationSource` exposes a [`voiceActivityFlag`](https://w3c.github.io/webrtc-pc/webrtc.html#dom-rtcrtpsynchronizationsource-voiceactivityflag) boolean, but this has not been implemented anywhere, nor am I aware that anyone is planning to implement this in the short term.

As a result, I think we should mark the feature as at risk for our next publication.

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/2496 using your GitHub account