Updated sphinx version to fix docs build failure

[vfdev-5]

Description:

• Updated sphinx version to fix docs build failure

Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/sphinx/builders/html/__init__.py", line 1127, in handle_page
    output = self.templates.render(templatename, ctx)
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/sphinx/jinja2glue.py", line 200, in render
    return self.environment.get_template(template).render(context)
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/jinja2/environment.py", line 1291, in render
    self.environment.handle_exception()
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/jinja2/environment.py", line 925, in handle_exception
    raise rewrite_traceback_stack(source=source)
  File "/opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/sphinx/themes/basic/page.html", line 10, in top-level template code
    {%- extends "layout.html" %}
  File "/home/runner/work/ignite/ignite/docs/source/_templates/layout.html", line 70, in top-level template code
    <link rel="preload" href="{{ pathto('_static/' + style, 1) }}" as="style" />
jinja2.exceptions.UndefinedError: 'style' is undefined

(https://github.com/pytorch/ignite/actions/runs/6424249673/job/17444472539?pr=3079)

Check list:

• New tests are added (if a new feature is added)
• New doc strings: description and/or example code are in RST format
• Documentation is updated (if required)