.readthedocs.yml: Ubuntu-24.04's default Python is 3.12

https://documentation.ubuntu.com/ubuntu-for-developers/reference/availability/python

Author: cclauss

.readthedocs.yml

@@ -2,9 +2,9 @@
 version: 2
 
 build:
-  os: ubuntu-22.04
+  os: ubuntu-24.04
   tools:
-    python: "3.8"
+    python: "3.12"
 
 formats:
   - htmlzip