fix: update heroku app.json to use premium plans (#5349)

* fix: update heroku app.json to use premium plans

Use premium/paid dynos and addons as Heroku is set to deprecate free dynos/addons.

* fix: set default stack to heroku-20

* chore: update heroku app.json to use new dyno types

web and worker to use basic dynos
redis and postgres to use mini

app.json

@@ -41,16 +41,24 @@
   "formation": {
     "web": {
       "quantity": 1,
-      "size": "FREE"
+      "size": "basic"
     },
     "worker": {
       "quantity": 1,
-      "size": "FREE"
+      "size": "basic"
     }
   },
   "stack": "heroku-20",
   "image": "heroku/ruby",
-  "addons": [ "heroku-redis", "heroku-postgresql"],
+  "addons": [
+    {
+      "plan": "heroku-redis:mini"
+    },
+    {
+      "plan": "heroku-postgresql:mini"
+    }
+  ],
+  "stack": "heroku-20",
   "buildpacks": [
     {
       "url": "heroku/ruby"