RTD maintenance (#7477)

* upgrade the os

* get the correct version in builds

Even if we don't display it in the built documentation anymore, some
functionality like `pandas.DataFrame.to_xarray` have a version check.

* print the last 5 commits [skip-rtd] [skip-ci]

* remove the "unshallow"

apparently, it does not make sense for complete repos, so git fails.

* also remove the history print

The idea was to check whether there was a special commit on top of the
most recent commit of the PR (this seems to be the case on GH
actions). There's none, so we don't need the print.

* [skip-ci] [skip-rtd]

* [skip-ci] [skip-rtd]

Author: keewis

.readthedocs.yaml

@@ -1,12 +1,14 @@
 version: 2
 
 build:
-  os: ubuntu-20.04
+  os: ubuntu-22.04
   tools:
     python: mambaforge-4.10
   jobs:
     post_checkout:
       - (git --no-pager log --pretty="tformat:%s" -1 | grep -vqF "[skip-rtd]") || exit 183
+    pre_install:
+      - git update-index --assume-unchanged doc/conf.py ci/requirements/doc.yml
 
 conda:
   environment: ci/requirements/doc.yml