Upgrade Sphinx version used in doc deployment

It was breaking with:
```
sphinx-build "spec/2021.12" "_site/2021.12" -W --keep-going
Running Sphinx v4.3.0

Sphinx version error:
The sphinxcontrib.applehelp extension used by this project needs at least Sphinx v5.0; it therefore cannot be built with this version.
```

Author: rgommers

doc-requirements.txt

@@ -1,8 +1,7 @@
-sphinx==4.3.0
+sphinx==6.2.1
 sphinx-material==0.0.30
 myst-parser
 sphinx_markdown_tables
 sphinx_copybutton
 sphinx_favicon
-docutils<0.18
 sphinx-math-dollar