Update Heroku stack

[jim]

Describe the bug

Production (and staging) are on older Heroku stacks. They will be supported for a while, but it makes sense to upgrade them sooner rather than later.

To Reproduce Steps to reproduce the behavior:

1. Deploy to staging or production
2. See this message:

   remote: This app is using the Heroku-18 stack, which is supported until April 30th, 2023.
   remote: A newer stack is available: Heroku-20. To upgrade, see:
   remote: https://devcenter.heroku.com/articles/upgrading-to-the-latest-stack
   

Expected behavior No deprecation message is shown.