Update Firefox support for RTCDataChannel features (#5593)
Update Firefox support for RTCDataChannel features (#5593)

* RTCDataChannel.onmessage property works in FF-71

* RTCDataChannel.onmessage versionAdded for FF is true

* update RTCDataChannel suppot matrix for FF and FFA

mdn/browser-compat-data,mdn/browser-compat-data,chrisdavidmills/browser-compat-data,chrisdavidmills/browser-compat-data