Here’s how to restore such a backup
https://meta.discourse.org/t/backup-failing-to-restore-cannot-restore-into-different-schema/88431/17
In general, as there were quite some of these issues in the 1.9.x versions, it’s best to first go to the latest Discourse version and then upgrade Postgres. Latest still works fine on 9.5, there is no need to upgrade Postgres first.
Oh, since you got it up and running again: to avoid this just run db:seed_fu before you backup or upgrade Postgres.