Update Heroku PostgreSQL Add-on Plan in Example Due to Removal of Free Tier

[89sundown]

Hello, there is no longer a free tier in Heroku. The below line causes the build to fail because this addon does not exist. You can see all the plans in the heroku cli heroku addons:plans heroku-postgresql. I updated my code to the below and was able to get it working.

https://github.com/hashicorp/learn-terraform-heroku/blob/16188171f369352205f3276a0247375f225844e6/main.tf#L13

resource "heroku_addon" "postgres" {
  app  = heroku_app.example.id
  plan = "heroku-postgresql:essential-0" <-- existing cheapest service
}