feature: use +100 staging bit for taproot channels

In this commit, we carry out a new notion introduced during a recent
spec meeting to use a feature bit plus 100 before the feature has been
finalized in the spec.

We split into the Final and Staging bits.

rpcserver.go

@@ -2152,7 +2152,7 @@ func (r *rpcServer) parseOpenChannelReq(in *lnrpc.OpenChannelRequest,
 
 		channelType = new(lnwire.ChannelType)
 		fv := lnwire.NewRawFeatureVector(
-			lnwire.SimpleTaprootChannelsRequired,
+			lnwire.SimpleTaprootChannelsRequiredStaging,
 		)
 
 		// TODO(roasbeef): no need for the rest as they're now