Fix padding bit in forwarded packet. (#4690)

Addresses https://github.com/livekit/livekit/issues/4689

We were probably missing a couple of bits with this
1. Not affected for regular traffic like from browsers as it does not
   add padding, but special clients were affected.
2. Probe packets were probably using wrong last byte as pion/rtp would
   have overwrriten with 0 because the header.PaddingSize for the newer
   versions were not set. That could have affected bandwidth estimation
   catch up.

pkg/sfu/downtrack.go

@@ -1116,7 +1116,7 @@ func (d *DownTrack) WriteRTP(extPkt *buffer.ExtPacket, layer int32) int32 {
 	hdr := RTPHeaderFactory.Get().(*rtp.Header)
 	*hdr = rtp.Header{
 		Version:        extPkt.Packet.Version,
-		Padding:        extPkt.Packet.Padding,
+		Padding:        false,
 		Marker:         tp.marker,
 		PayloadType:    d.getTranslatedPayloadType(extPkt.Packet.PayloadType),
 		SequenceNumber: uint16(tp.rtp.extSequenceNumber),
@@ -1307,6 +1307,7 @@ func (d *DownTrack) WritePaddingRTP(bytesToSend int, paddingOnMute bool, forceMa
 		*hdr = rtp.Header{
 			Version:        2,
 			Padding:        true,
+			PaddingSize:    byte(RTPPaddingMaxPayloadSize),
 			Marker:         false,
 			PayloadType:    uint8(d.payloadType.Load()),
 			SequenceNumber: uint16(snts[i].extSequenceNumber),
@@ -2185,7 +2186,7 @@ func (d *DownTrack) retransmitPacket(epm *extPacketMeta, sourcePkt []byte, isPro
 	hdr := RTPHeaderFactory.Get().(*rtp.Header)
 	*hdr = rtp.Header{
 		Version:        pkt.Header.Version,
-		Padding:        pkt.Header.Padding,
+		Padding:        false,
 		Marker:         epm.marker,
 		PayloadType:    d.getTranslatedPayloadType(pkt.Header.PayloadType),
 		SequenceNumber: epm.targetSeqNo,
@@ -2375,6 +2376,7 @@ func (d *DownTrack) WriteProbePackets(bytesToSend int, usePadding bool) int {
 			*hdr = rtp.Header{
 				Version:        2,
 				Padding:        true,
+				PaddingSize:    byte(RTPPaddingMaxPayloadSize),
 				Marker:         false,
 				PayloadType:    rtxPT,
 				SequenceNumber: uint16(rtxExtSequenceNumber),