Move RTT errors to Debugw. (#2742)

With the move to forwarding NTP timestamp as is, we get a bunch more of
this error logged as the remote is basing it off of previous report and
local (i. e. server-side) bases it off of a more recent report.

Anyhow, this code has been around for a long time and there is nothing
new to learn from those errors. Just log it at Debugw in case we can
learn something from it for specific projects or environments where
Debugw is okay.

pkg/sfu/buffer/rtpstats_sender.go

@@ -491,9 +491,7 @@ func (r *RTPStatsSender) UpdateFromReceiverReport(rr rtcp.ReceptionReport) (rtt
 		if err == nil {
 			isRttChanged = rtt != r.rtt
 		} else {
-			if !errors.Is(err, mediatransportutil.ErrRttNotLastSenderReport) && !errors.Is(err, mediatransportutil.ErrRttNoLastSenderReport) {
-				r.logger.Warnw("error getting rtt", err)
-			}
+			r.logger.Debugw("error getting rtt", "error", err)
 		}
 	}