peer+server: downgrade to legacy coop close for taproot channels

In this commit, we implement logic to downgrade to the legacy coop close
for taproot channels. Before this commit, we wouldn't allow nodes to
start up with both the taproot flag and the rbf flag activated.

In the future, once we implement the spec updates, we'll add support for
this combo, and can revert parts of this commit.

server.go

@@ -612,17 +612,6 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 			"aux controllers")
 	}
 
-	// For now, the RBF coop close flag and the taproot channel type cannot
-	// be used together.
-	//
-	// TODO(roasbeef): fix
-	if cfg.ProtocolOptions.RbfCoopClose &&
-		cfg.ProtocolOptions.TaprootChans {
-
-		return nil, fmt.Errorf("RBF coop close and taproot " +
-			"channels cannot be used together")
-	}
-
 	//nolint:ll
 	featureMgr, err := feature.NewManager(feature.Config{
 		NoTLVOnion:                cfg.ProtocolOptions.LegacyOnion(),