replace build.image with build.os in the .readthedocs.yaml config (#2215)

https://docs.readthedocs.io/en/stable/config-file/v2.html

build.image will be deprecated in October 2023

.readthedocs.yaml

@@ -9,6 +9,10 @@ sphinx:
 formats: [htmlzip]
 
 python:
-   version: "3.8"
    install:
    - requirements: docs/.sphinx/requirements.txt
+
+build:
+   os: ubuntu-20.04
+   tools:
+      python: "3.8"