Set Heroku stack to heroku-16 in app.json

The graphviz buildpack does not support the new default `heroku-18`
stack yet. Until it does we have to fall back to the older stack.

 #2377

app.json

@@ -21,5 +21,6 @@
       "postdeploy": "bundle exec rake db:migrate"
     },
     "addons": ["heroku-postgresql"],
-    "success_url": "/users/sign_up"
+    "success_url": "/users/sign_up",
+    "stack": "heroku-16"
 }