Use Ubuntu 20.04.

If we don’t do it in January2021 (early Dawn), the next slot would be post-Dawn, e.g. in July 2021. Generally these changes are best done early in a release.

A key difference is how it treats python 3 vs 2. I understand 20.04 uses python 3 as default. A newer gcc version (9.3 vs 7.5) might generally be useful. Ubuntu 18.04, though, will still be supported for a long time. So it’s primarily about using new features (than security fixes).  

Not strictly linked we coudl also update to golang 1.14 in the base image.