Clear announcement_sigs on FundingScope promotion

When a splice transaction is promoted (i.e., when splice_locked has been
exchanged), announcement_signatures must be sent. However, if we try to
send a channel_announcement before they are received, then the
signatures will be incorrect. To avoid this, clear the counterparty's
announcement_signatures upon promoting a FundingScope.

Author: jkczyz

lightning/src/ln/channel.rs

@@ -6437,6 +6437,7 @@ macro_rules! promote_splice_funding {
 		core::mem::swap(&mut $self.funding, $funding);
 		$self.interactive_tx_signing_session = None;
 		$self.pending_splice = None;
+		$self.context.announcement_sigs = None;
 		$self.context.announcement_sigs_state = AnnouncementSigsState::NotSent;
 
 		// The swap above places the previous `FundingScope` into `pending_funding`.