.readthedocs.yml: update Python version and Ubuntu release

The current documentation build dependencies are not compatible with
Python 3.9. Update to Python 3.12.

Use the current Ubuntu LTS release, i.e. 24.04 Noble Numbat.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/.readthedocs.yml b/.readthedocs.yml
index 7c6c255..16418f2 100644
--- a/.readthedocs.yml
+++ b/.readthedocs.yml
@@ -6,11 +6,11 @@
 version: 2
 
 build:
-  os: "ubuntu-20.04"
+  os: "ubuntu-24.04"
   apt_packages:
     - python3-six
   tools:
-    python: "3.9"
+    python: "3.12"
 
 # Build documentation in the docs/ directory with Sphinx
 sphinx: