Update Ubuntu actions image to 22.04 or later

[StevenMaude]

The Ubuntu 20.04 image is deprecated.

This in itself is easy to change, but Ubuntu 22.04 and later don't have a version of Python 3.6 available (neither does deadsnakes).

So:

• either the workflow has to be modified to get Python 3.6 installed via another route (pyenv? — uv won't do this because the standalone builds it uses look like there has never been a Python 3.6 available)
• or the Python version needs to be updated to 3.7 or later (and 3.7, and 3.8 are also end-of-life)