Fix readthedocs fail test changing build.image into build.os

[YannCabanes]

The readthedocs test is failing on the main branch with the following error message:
The configuration key "build.image" is deprecated. Use "build.os" instead to continue building your project. Read more at https://docs.readthedocs.io/en/stable/config-file/v2.html#build-os

Indeed, build.image is deprecated since Monday, October 16, 2023 and is now replaced by build.os:
https://blog.readthedocs.com/use-build-os-config/

Specific details to write the .readthedocs.yml file can be found at:
https://docs.readthedocs.io/en/stable/config-file/v2.html

[YannCabanes]

We have the following error related to the Ubuntu version:
Problem in your project's configuration. Invalid configuration option "build.os": expected one of (ubuntu-20.04, ubuntu-22.04), got ubuntu-latest

Here is a discussion about ubuntu-latest:
readthedocs/readthedocs.org#8861

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (37018ab) 93.32% compared to head (80a4cd9) 93.32%.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #487   +/-   ##
=======================================
  Coverage   93.32%   93.32%           
=======================================
  Files          67       67           
  Lines        5725     5725           
=======================================
  Hits         5343     5343           
  Misses        382      382           

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