Ubuntu 20.04 actions runner image deprecation

[dalle]

See actions/runner-images#11101

Ubuntu 20.04 actions runners are deprecated since 2025-02-01, and will be fully unsupported by 2025-04-01.

We have tests for that use these images to check that we are backwards compatible.

[lemire]

I think that we have no choice but to drop them. :-/