multi: extract new DeriveHeightHint() function, use for new rbf closer

With this commit, we make sure we set the right height hint, even if the
channel is a zero conf channel.

peer/brontide.go

@@ -3857,7 +3857,7 @@ func (p *Brontide) initRbfChanCloser(
 	spendEvent := protofsm.RegisterSpend[chancloser.ProtocolEvent]{
 		OutPoint:   channel.ChannelPoint(),
 		PkScript:   channel.FundingTxOut().PkScript,
-		HeightHint: scid.BlockHeight,
+		HeightHint: channel.DeriveHeightHint(),
 		PostSpendEvent: fn.Some[chancloser.RbfSpendMapper](
 			chancloser.SpendMapper,
 		),