PG17 compatibility: fix pg16 to pg17 upgrade

In citus_prepapre_upgrade(), don't drop any_value when upgrading from
PG16+, because PG16+ has its own any_value function. Attempting to do
so results in an error. When 16 becomes the minimum supported Postgres
version, the drop statements can be removed.

src/backend/distributed/sql/udfs/citus_prepare_pg_upgrade/latest.sql

@@ -18,13 +18,15 @@ BEGIN
     DROP AGGREGATE IF EXISTS array_cat_agg(anyarray);
     DROP AGGREGATE IF EXISTS array_cat_agg(anycompatiblearray);
 
-    -- We should drop any_value because PG16 has its own any_value function
+    -- We should drop any_value because PG16+ has its own any_value function
     -- We can remove this part when we drop support for PG16
-    DELETE FROM pg_depend WHERE
-        objid IN (SELECT oid FROM pg_proc WHERE proname = 'any_value' OR proname = 'any_value_agg') AND
-        refobjid IN (select oid from pg_extension where extname = 'citus');
-    DROP AGGREGATE IF EXISTS pg_catalog.any_value(anyelement);
-    DROP FUNCTION IF EXISTS pg_catalog.any_value_agg(anyelement, anyelement);
+    IF substring(current_Setting('server_version'), '\d+')::int < 16 THEN
+        DELETE FROM pg_depend WHERE
+            objid IN (SELECT oid FROM pg_proc WHERE proname = 'any_value' OR proname = 'any_value_agg') AND
+            refobjid IN (select oid from pg_extension where extname = 'citus');
+        DROP AGGREGATE IF EXISTS pg_catalog.any_value(anyelement);
+        DROP FUNCTION IF EXISTS pg_catalog.any_value_agg(anyelement, anyelement);
+    END IF;
 
     --
     -- Drop existing backup tables