ci: update documentation build to ubuntu 24.04

[shiftinv]

Summary

The docs workflow previously used ubuntu-20.04 to be more consistent with the readthedocs build. GitHub is deprecating ubuntu-20.04 in April, with brownouts starting in March: https://github.blog/changelog/2025-01-15-github-actions-ubuntu-20-runner-image-brownout-dates-and-other-breaking-changes/.
This updates both to use 24.04 instead.

Checklist

• If code changes were made, then they have been tested
  • I have updated the documentation to reflect the changes
  • I have formatted the code properly by running pdm lint
  • I have type-checked the code by running pdm pyright
• This PR fixes an issue
• This PR adds something new (e.g. new method or parameters)
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)