Upgrade to heroku-20 stack

app.json

@@ -4,7 +4,7 @@
   "scripts": {
     "postdeploy": "python manage.py migrate && python manage.py generateSampleData"
   },
-  "stack": "heroku-18",
+  "stack": "heroku-20",
   "env": {
     "DEBUG": {
       "required": true
@@ -51,4 +51,4 @@
       "url": "heroku/python"
     }
   ]
-}
+}