use ubuntu-20.04 per @bnavigator comment

murrayrm · murrayrm · commit f0820c6d9c75 · 2022-12-18T08:08:02.000-08:00
diff --git a/.readthedocs.yaml b/.readthedocs.yaml
@@ -7,7 +7,7 @@ version: 2
 
 # Set the version of Python and other tools you might need
 build:
-  os: ubuntu-20.04
+  os: ubuntu-22.04
   tools:
     python: "3.9"
 
