Update Heroku stack version

[renaudjenny]

Message from the last Heroku deploy:

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.
Please upgrade to a newer stack as soon as possible:
https://devcenter.heroku.com/articles/upgrading-to-the-latest-stack
https://help.heroku.com/0S5P41DC/heroku-16-end-of-life-faq