webrtcsink: set rtpgccbwe min bitrate
Sets the min bitrate property on webrtcsink's rtpgccbwe element. As the max-bitrate is multiplied by the number of encoders, I've kept that conform for the min-bitrate as well.
For me it's not really clear from an application developer perspective that this multiplication is happening though; would it be useful to change the properties descriptions to reflect that?