A few months ago, we released a new build image based on Ubuntu 20.04 “Focal Fossa” as an opt-in beta. Today, it graduates out of beta, as the default for all new sites! 
For existing sites, you can still switch to the new build image by visiting your site dashboard and navigating to Site settings > Build & deploy > > Continuous Deployment > Build image selection (more info in docs).
For more information about migrating sites to Focal, and troubleshooting any errors you might find along the way, visit our migration guide.