multi: rename AnnounceSignatures to AnnounceSignatures1

In preparation for adding a new message, AnnounceSignatures2 along with
an AnnounceSignatures interface, we rename the existing message to
AnnounceSignatures1.

funding/manager_test.go

@@ -1297,7 +1297,7 @@ func assertAnnouncementSignatures(t *testing.T, alice, bob *testNode) {
 		gotNodeAnnouncement := false
 		for _, msg := range announcements {
 			switch msg.(type) {
-			case *lnwire.AnnounceSignatures:
+			case *lnwire.AnnounceSignatures1:
 				gotAnnounceSignatures = true
 			case *lnwire.NodeAnnouncement:
 				gotNodeAnnouncement = true