You can add code to your app.yml to update postgres to the matching version. I’ve done it before, but can’t find my notes. The easiest thing to do is go into the container, issue the apt commands to get the right postgres clients installed and then try the backup. Once that works, you add those commands to exec commands in your app.yml so that it’ll work after the next rebuild.
If you need someone to walk you though it, you can email me or post in Marketplace.