Move most CI jobs to a recent Ubuntu

[gilles-peskine-arm]

We're currently running most CI jobs on Ubuntu 16.04, which has been out of support for a while. Migrate to a recent Ubuntu LTS. We've been talking about moving to 20.04, but it's taken us long enough that we should probably move to 22.04 now.

We may want to keep a few jobs running on an older system, so as to test with older versions of Python, compilers and libc. Alternatively, we may decide to use a single Docker image and just install older Python and older compilers there.

Definition of done for this issue:

• We have a strategy for doing most testing on a recent platform, but still run some tests on older platforms.
• all.sh passes on the recent platform.
• The Jenkins job dispatch defaults to the recent Ubuntu platform, with older platforms for explicitly listed jobs only.
• If we decide to completely retire older platforms (Ubuntu 16.04, maybe also 18.04), remove the Docker images.

Prerequisites:

• Migrate abi_check to Ubuntu 20.04 or 22.04 #57
• Possibly Create Ubuntu 22.04 Docker image #58
• Possibly additional tasks to be created if all.sh doesn't pass on the chosen Ubuntu version