Documentation build fails

[kilianvolmer]

Bug description

The documentation build by readthedocs fails with

 Config file validation error

Config validation error in build.os. Expected one of (ubuntu-20.04, ubuntu-22.04, ubuntu-24.04, ubuntu-lts-latest), got type str (ubuntu-latest). Double check the type of the value. A string may be required (e.g. "3.10" instead of 3.10)
``

### Version

Any

### To reproduce

See https://app.readthedocs.org/projects/memilio/builds/?version__slug=latest

### Relevant log output

```zsh

Add any relevant information, e.g. used compiler, screenshots.

Checklist

• Attached labels, especially loc:: or model:: labels.
• Linked to project