Heroku-16 stack upgrade

[gburgett]

This app is using the Heroku-16 stack, which is deprecated:
https://devcenter.heroku.com/changelog-items/1958

From May 1st 2021, Heroku-16 will be end-of-life, and apps using it
will no longer receive security updates, and be run at your own risk.
From June 1st 2021, builds will no longer be allowed for Heroku-16 apps.