[doc] Upgrade Read the Docs build image to Ubuntu 24.04/Python 3.13 (#10945)

Ubuntu 20.04 is being deprecated by Read the Docs (June 2026), might as well upgrade this one to a more modern Ubuntu and a faster python.

Author: Pierre-Sassoulas

.readthedocs.yaml

@@ -11,9 +11,9 @@ python:
     - requirements: doc/readthedoc_requirements.txt
 
 build:
-  os: ubuntu-22.04
+  os: ubuntu-26.04
   tools:
-    python: "3.11"
+    python: "3.14"
   jobs:
     pre_build:
       - towncrier build --yes --date TBA