Update RTD config for build.os config change (#9272)

* Update RTD config for build.os config change

See https://blog.readthedocs.com/use-build-os-config/ for details. We are in a temporary brownout with the old config key disabled, so RTD builds will be failing for the next few hours; but the change will become permanent next month.

* Remove old python version key

It's now in the build section.

* Downgrade Python back to 3.8

Otherwise need to update requirements.txt, which is beyond my pay grade.

.readthedocs.yml

@@ -3,7 +3,11 @@ version: 2
 sphinx:
   configuration: doc/conf.py
 
+build:
+  os: "ubuntu-22.04"
+  tools:
+    python: "3.8"
+
 python:
-  version: 3.8
   install:
     - requirements: doc/requirements.txt