Upgrade heroku stack

[hanneskaeufler]

Depending on where we go with the site next (static app, ...), if we decide to keep the same stack for now, we have to upgrade the heroku "stack". We get

This application is still using the Cedar-14 stack for which the end-of-life window began May 1, 2019:
https://devcenter.heroku.com/changelog-items/1603
The end-of-life window ends on May 1, 2020 after which Cedar-14 will no longer
receive security updates and Cedar-14 app builds will be disabled.
Please upgrade to the latest stack by following these instructions:
https://devcenter.heroku.com/articles/upgrading-to-the-latest-stack