Remove public schema dependency for 10.1 upgrades

This commit contains a subset of the changes that should be cherry
picked to 10.1 releases.

(cherry picked from commit efc5776451)

src/backend/distributed/sql/citus--10.0-4--10.1-1.sql

@@ -1,4 +1,4 @@
--- citus--10.0-3--10.1-1
+-- citus--10.0-4--10.1-1
 
 -- add the current database to the distributed objects if not already in there.
 -- this is to reliably propagate some of the alter database commands that might be

src/backend/distributed/sql/downgrades/citus--10.1-1--10.0-4.sql

@@ -1,4 +1,8 @@
--- citus--10.1-1--10.0-3
+-- citus--10.1-1--10.0-4
+
+-- This migration file aims to fix the issues with upgrades on clusters without public schema.
+
+-- copy of citus--10.1-1--10.0-3
 
 -- remove databases as distributed objects to prevent unknown object types being managed
 -- on older versions.