Remove Ubuntu 20.04 CI Runners

[folmos-at-orange]

Description

The Ubuntu 20.04 runners are deprecated and will be fully removed by 2025-04-01. Note that Github will start randomly failing jobs on these runner to raise awareness.

Questions/Ideas

• Remove the ubuntu-20.04 runners from the CI matrices
• We can use a Docker image to continue testing on this platform.

[popescu-v]

Bumped priority as the deadline approaches and parts of the CI would not (fully) run anymore unless this issue is done.