[CI] fix Ubuntu 20.04 images using Ubuntu 18.04

[cz4rs]

magistrate's ubuntu-20.04 Dockerfiles are actually symlinks to ubuntu-18.04 ones, see https://github.com/DARMA-tasking/magistrate/tree/develop/ci/docker.

• create Dockerfiles using Ubuntu 20.04
• Dockerfiles can be parametrized with Ubuntu version as is done in vt (see https://github.com/DARMA-tasking/vt/blob/develop/ci/docker/ubuntu-gnu-cpp.dockerfile) so we can avoid having multiple files
• consider removing builds based on Ubuntu 18.04 (not supported anymore, might still be necessary for older compilers)
  • Ubuntu 20.04 provides gcc-8, so we don't need 18.04 in CI unless some users still run it
• consider adding builds based on Ubuntu 24.04 (this is the current latest)
• vt is in the process of removing gcc-8 support, remove it in magistrate as well