Filter by templated schema name for postgres queries (#21635)

There were certain queries which did not filter
by the current schema, but hardcoded public schema.

Apart from that, we always filter with the current schema
which allows connection proxies to run multiple MM within
a single DB.
```release-note
NONE
```

db/migrations/postgres/000046_create_users.up.sql

@@ -54,6 +54,7 @@ BEGIN
 SELECT count(*) != 0 INTO column_exist
     FROM information_schema.columns
     WHERE table_name = 'users'
+    AND table_schema = current_schema()
     AND column_name = 'roles'
     AND NOT data_type = 'varchar(256)';
 IF column_exist THEN