.readthedocs.yml: Add build: entry to avoid old docker images

This follows the advice from readthedocs/readthedocs.org#10290 (comment) to address urllib3/urllib3#2168.

.readthedocs.yml

@@ -5,6 +5,9 @@
 # Required
 version: 2
 
+build:
+  os: ubuntu-22.04
+
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
   configuration: documentation/source/conf.py