Update CI build image to Ubuntu 20.04

[jcoupey]

CI runs currently use Ubuntu bionic images, which means we are checking the build against compiler versions that are (much) older than the ones currently available out of the box in recent dev/production boxes. Switching to Ubuntu focal should enable updating:

• gcc from version 8 to 9;
• clang from version 6 to 10.

The way to do this is somehow related to the outcome of #436 but I'm still filling a dedicated ticket as we'll need it in any case at some point.

[jcoupey]

Done in #517.