add upgrade and downgrade path

src/backend/distributed/sql/citus--12.1-1--13.0-1.sql

@@ -0,0 +1,3 @@
+-- citus--12.1-1--13.0-1.sql
+
+-- bump version to 13.0-1

src/backend/distributed/sql/downgrades/citus-13.0-1--12.1-1.sql

@@ -0,0 +1,2 @@
+-- citus--13.0-1--12.1-1
+-- this is an empty downgrade path since citus--12.1-1--13.0-1.sql is empty