Resolve "Upgrade testing image to use Ubuntu 20.04"

What does this MR do?

This MR upgrades the testing image to Ubuntu 20.04.

• Uses ubuntu:focal image for utopia-base:focal-v1 image
• Adapt dependencies
  • Use gcc-10 now
  • Need libstdc++-dev for clang
• Updates README
  • Update installation instructions for Ubuntu 20.04
  • Adds a comment that Utopia is tested against 20.04, but that enforcements are those of 19.10

Is there something that needs to be double checked?

• Should we add a timeline for the enforcements right into the README and link to #190 (closed) there?
• Any other idea for the tzdata workaround? (Not critical, though, imo)

Can this MR be accepted?

• Implemented the changes
• gcc-10 and clang v10 is detected and used by Utopia
• Added or updated documentation
• Reasonably up-to-date with current master
• Pipeline passing without warnings
• History cleaned up
• Approved by @peanutfun (anyone else?)

Related issues

Meta-task #190 (closed)