Update ReadTheDocs build image from Ubuntu 20.04 to 24.04

[rly]

Summary

ReadTheDocs is deprecating the Ubuntu 20.04 LTS build image (build.os: ubuntu-20.04). After June 1, 2026, projects still using this image will fail to build.

Ubuntu 20.04 LTS is no longer receiving security updates and ReadTheDocs considers it insecure to continue offering.

Current state

In .readthedocs.yaml:

build:
  os: ubuntu-20.04

Required change

Update to a supported image (recommended ubuntu-24.04):

build:
  os: ubuntu-24.04

Supported options: ubuntu-22.04, ubuntu-24.04, ubuntu-lts-latest.

See: https://docs.readthedocs.io/en/stable/config-file/v2.html#build-os