Upgrade Heroku stack

[cben]

“Your Heroku account has one or more apps running on the Cedar-14 stack, which was deprecated on May 3, 2018. You must upgrade your apps to the latest stack, which is currently Heroku-18.

We communicated that the end-of-life (EOL) window began on May 1, 2019 in accordance with Heroku's stack policy. We are providing you a grace period of 1 year that began on May 1, 2019 during which your apps on Cedar-14 will still run, build, and receive critical security updates.

Key dates:

May 3, 2018: Heroku announced deprecation of Cedar-14 for April 2019. May 1, 2019: EOL window for Cedar-14 began. Apps on Cedar-14 will continue to run, build, and receive security updates. Apps and builds running on later stacks will not be affected. May 1, 2020: EOL window for Cedar-14 will end. Apps on Cedar-14 will continue to run, but may not build or receive security updates. Apps running on later stacks will not be affected. What do I need to do? Please use this Dev Center guide to upgrade your apps on Cedar-14 to the latest stack.

What apps do I have on the Cedar-14 stack? Your account has the following apps running on Cedar-14:

mathdown, mathdown-staging”