Use recommended `heroku/builder:22`

Switch to the recommended heroku/builder:22 image.

https://github.com/heroku/cnb-builder-images#available-images

Release notes

As the heroku/builder:20 image used by the Auto Build component of Auto DevOps was deprecated upstream, we are moving to the heroku/builder:22 image. The new images are based on ubuntu:22.04 instead of ubuntu:20.04 and the respective packages. Otherwise, they support the same version 0.18.x cloud native buildpack lifecycle APIs.

This breaking change arrives outside a GitLab major release as we need to accommodate a breaking change upstream and the upgrade is unlikely to break app developers' pipelines. If needed, users can manually configure to user the heroku/builder:20 and skip the errors to revert the changes.