Update Heroku stack

[daterdots]

remote: This app is using the Heroku-18 stack, which is deprecated:
remote: https://devcenter.heroku.com/changelog-items/2432
remote:
remote: From April 30th 2023, Heroku-18 will be end-of-life, and apps using it
remote: will no longer receive security updates, and be run at your own risk.
remote:
remote: From May 1st 2023, builds will no longer be allowed for Heroku-18 apps.
remote:
remote: Please upgrade to a newer stack as soon as possible:
remote: https://devcenter.heroku.com/articles/upgrading-to-the-latest-stack
remote: https://help.heroku.com/X5OE6BCA/heroku-18-end-of-life-faq