Fix cagg_migrate_to_time_bucket during update

In #6837 we added a migration for CAggs using `time_bucket_ng` to use
the new version of `time_bucket` that support orign and/or offset.

We made a mistake because we place the code to migrate existing CAggs
using `time_bucket_ng` to `time_bucket` in the update script but this
code should be placed instead on post-update.sql script where we can
call functions from the new extension version loaded.

sql/updates/2.14.2--2.15.0.sql

@@ -427,23 +427,3 @@ DROP FUNCTION IF EXISTS _timescaledb_functions.policy_job_error_retention_check(
 --
 -- END bgw_job_stat_history
 --
-
--- Migrate existing CAggs using time_bucket_ng to time_bucket
-CREATE PROCEDURE _timescaledb_functions.cagg_migrate_to_time_bucket(cagg REGCLASS)
-   AS '@MODULE_PATHNAME@', 'ts_continuous_agg_migrate_to_time_bucket' LANGUAGE C;
-
-DO $$
-DECLARE
-  cagg_name regclass;
-BEGIN
-  FOR cagg_name IN
-    SELECT pg_catalog.format('%I.%I', user_view_schema, user_view_name)::regclass
-      FROM _timescaledb_catalog.continuous_agg cagg
-      JOIN _timescaledb_catalog.continuous_aggs_bucket_function AS bf ON (cagg.mat_hypertable_id = bf.mat_hypertable_id)
-      WHERE
-         bf.bucket_func::text LIKE '%time_bucket_ng%'
-  LOOP
-    CALL _timescaledb_functions.cagg_migrate_to_time_bucket(cagg_name);
-  END LOOP;
-END
-$$;

sql/updates/post-update.sql

@@ -171,3 +171,22 @@ $$;
 -- Repair relations that have relacl entries for users that do not
 -- exist in pg_authid
 CALL _timescaledb_functions.repair_relation_acls();
+
+-- Migrate existing CAggs using time_bucket_ng to time_bucket
+DO $$
+DECLARE
+  cagg_name regclass;
+BEGIN
+  FOR cagg_name IN
+    SELECT
+      pg_catalog.format('%I.%I', user_view_schema, user_view_name)::regclass
+    FROM
+      _timescaledb_catalog.continuous_agg
+      JOIN _timescaledb_catalog.continuous_aggs_bucket_function USING (mat_hypertable_id)
+    WHERE
+      bucket_func::text LIKE '%time_bucket_ng%'
+  LOOP
+    CALL _timescaledb_functions.cagg_migrate_to_time_bucket(cagg_name);
+  END LOOP;
+END
+$$;