Upgrade to New Heroku Plan (#624)

As announced on May 1, 2024, Heroku Postgres mini and basic plans will
reach end-of-life (EOL) on May 22, 2024.
Heimdall was using heroku-postgresql:mini.

Make the necessary changes in the app.json, update to the new Essential
plan to:
heroku-postgresql:mini → heroku-postgresql:essential-0

app.json

@@ -11,7 +11,7 @@
   },
   "environments": {
     "review": {
-      "addons": ["heroku-postgresql:hobby-basic"]
+      "addons": ["heroku-postgresql:essential-0"]
     }
   }
 }