docs: update .readthedocs.yaml

When we specify build.os in this config, we don't start out with
pre-installed sphinx versions etc and can won't have to worry about
issues caused by pre-installed versions of sphinx etc.

.readthedocs.yaml

@@ -1,31 +1,19 @@
 # Configuration on how ReadTheDocs (RTD) builds our documentation
 # ref: https://readthedocs.org/projects/jupyterhub-kubespawner/
 # ref: https://docs.readthedocs.io/en/stable/config-file/v2.html
-
-# Required (RTD configuration version)
+#
 version: 2
 
-build:
-  os: ubuntu-20.04
-  tools:
-    python: "3.8"
-
 sphinx:
   configuration: docs/source/conf.py
 
-# Optionally build docs in additional formats such as PDF and ePub
-formats: []
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.10"
 
-# Optionally set requirements required to build your docs
 python:
   install:
-    # WARNING: This requirements file will be installed without the pip
-    #          --upgrade flag in an existing environment. This means that if a
-    #          package is specified without a lower boundary, we may end up
-    #          accepting the existing version.
-    #
-    #          ref: https://github.com/readthedocs/readthedocs.org/blob/0e3df509e7810e46603be47d268273c596e68455/readthedocs/doc_builder/python_environments.py#L335-L344
-    #
     - requirements: docs/requirements.txt
     # NOTE:    We also install the kubespawner package itself, because we use
     #          the autodoc_traits sphinx extension to generate documentation via