Tracking issue for upgrading to `heroku-18` stack

[jtgeibel]

The heroku-16 stack is supported through April 2021 so we should migrate to heroku-18 before then. At this time (September 2020), the heroku-20 stack has not reached general availability.

Checklist

• Migrate staging to 18.04
• Migrate production to 18.04
• Bump CI stack from xenial to bionic #1867 - Migrate CI to 18.04

Original Migration Details

I'm opening this issue to document the process of bumping to the heroku-18 stack (from heroku-16) on my staging environment.

After bumping to the newest stack in the Heroku GUI, the changes take effect on the next deploy. The next build then failed with a linking error, due to missing openssl symbols. The build was using cached build files that were built on an older xenial-based system.

Following the instructions on the help page, I used the following commands clear the build cache and force a new deploy:

$ heroku plugins:install heroku-repo
$ heroku repo:purge_cache -a $appname
$ git commit --allow-empty -m "Force new deploy on heroku-18"
$ git push heroku master

After a clean build the site appears to operate normally, however I haven't yet tested all of the functionality, such as publishing.

If we decide to bump to the heroku-18 stack, then we'll also want to bump our CI to bionic to match.