invoice: bump year for new databases

Change-Id: I88918b103e7b128d5fc263873ce9d2ec9a739bd7
diff --git a/bgpwtf/invoice/model.go b/bgpwtf/invoice/model.go
index fcf4aba..548d8ff 100644
--- a/bgpwtf/invoice/model.go
+++ b/bgpwtf/invoice/model.go
@@ -75,7 +75,7 @@
 			invoice_id, final_uid, sealed_time
 		) values (
 			?,
-			( select printf("%04d", ifnull( (select final_uid as v from invoice_seal order by final_uid desc limit 1), 20000) + 1 )),
+			( select printf("%04d", ifnull( (select final_uid as v from invoice_seal order by final_uid desc limit 1), 21000) + 1 )),
 			?
 		)
 	`