Update readthedocs config to use build.os

The ReadTheDocs team deprecated the older build.image
config scheme. See this https://blog.readthedocs.com/use-build-os-config/
blog post for more information. This commit updates
our .yaml config file to use build.os

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

.readthedocs.yaml

@@ -4,6 +4,10 @@
 
 # Required
 version: 2
+build:
+   os: ubuntu-22.04
+   tools:
+      python: "3.11"
 
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
@@ -15,6 +19,5 @@ sphinx:
 
 # Optionally declare the Python requirements required to build your docs
 python:
-   version: 3.8
    install:
-   - requirements: doc/requirements.txt
+   - requirements: doc/requirements.txt