Update Read the Docs configuration for Ubuntu and Python

[AdamRJensen]

Message from readthedocs:

We are announcing the deprecation of the Ubuntu 20.04 LTS build image (build.os: ubuntu-20.04) on Read the Docs. After June 1, 2026, projects still using this image will fail when building until they are updated to a supported Ubuntu image.

This change keeps our build images current and helps us prepare for Ubuntu 26.04 LTS support. Ubuntu 20.04 LTS is now an older base image and is no longer receiving security updates, so we consider it insecure to continue to offer.

What do I need to do?
Update your .readthedocs.yaml to use a newer supported image. For most projects, we recommend:

build:
 os: ubuntu-24.04

Read the Docs currently supports ubuntu-22.04, ubuntu-24.04, and ubuntu-lts-latest for build.os. See all options in the docs: https://docs.readthedocs.io/en/stable/config-file/v2.html#build-os