Update sphinx in CI

[m-aguena]

The sphinx was set to v8.2.3 in CI, when a new version is run, we get the following error:

Theme error!

Versions
========

* Platform:         linux; (Linux-6.11.0-1018-azure-x86_64-with-glibc2.39)
* Python version:   3.13.9 (CPython)
* Sphinx version:   9.0.1
* Docutils version: 0.22.3
* Jinja2 version:   3.1.6
* Pygments version: 2.19.2

Last Messages
=============

    Writing evaluated template result to /home/runner/work/CLMM/CLMM/docs/_build/html/_static/basic.css
    Writing evaluated template result to /home/runner/work/CLMM/CLMM/docs/_build/html/_static/documentation_options.js
    copying static files: done
    copying extra files...
    copying extra files: done
    copying assets: done

    writing output... [  2%]
    api

Loaded Extensions
=================

* sphinx.ext.mathjax (9.0.1)
* alabaster (1.0.0)
* sphinxcontrib.applehelp (2.0.0)
* sphinxcontrib.devhelp (2.0.0)
* sphinxcontrib.htmlhelp (2.1.0)
* sphinxcontrib.serializinghtml (1.1.10)
* sphinxcontrib.qthelp (2.0.0)
* sphinx.ext.autodoc (9.0.1)
* sphinx.ext.autosummary (9.0.1)
* sphinx.ext.viewcode (9.0.1)
* sphinx.ext.napoleon (9.0.1)
* sphinx.ext.githubpages (9.0.1)
* IPython.sphinxext.ipython_console_highlighting (unknown version)

* sphinx_rtd_theme (unknown version)

Traceback
=========

      File "/usr/share/miniconda/lib/python3.13/site-packages/sphinx/builders/html/__init__.py", line 1223, in handle_page
        raise ThemeError(msg % self.config.html_theme) from None
    sphinx.errors.ThemeError: The 'sphinx_rtd_theme' theme does not support this version of Sphinx, because it uses the 'style' field in HTML templates, which was  was deprecated in Sphinx 5.1 and removed in Sphinx 7.0. The theme must be updated to use the 'styles' field instead. See https://www.sphinx-doc.org/en/master/development/html_themes/templating.html#styles