Remove deprecated theme configuration

For the configured Awesome Sphinx Theme [1] highlighting extension, the
sphinx-build currently yields the following diagnostics:

     WARNING: while setting up extension sphinxawesome_theme.highlighting: \
     You no longer have to include the `sphinxawsome_theme.highlighting` \
     extension. This extension will be removed in the next major release.

(via `make html`, the configuration file is `source/config.py`.)

The diagnostic message was introduced by sphinxawesome-theme 5.2.0,
  released May 31, 2024. [2], [3]

Removing the extension from the list of extensions in the configuration
file levitates.

No changes to requirements.txt, the extension was transitive as bundled
by the Awesome Sphinx Theme [1], and 5.2.0 deprecates it with the new
feature to "Support `pygments_style_dark` option that allows you to set
a different syntax highlighting scheme in light and dark modes." [3]

[1]: https://sphinxawesome.xyz/ "Awesome Sphinx Theme — Create functional and beautiful websites for your documentation with Sphinx."
[2]: https://pypi.org/project/sphinxawesome-theme/5.2.0/#history
[3]: https://github.com/kai687/sphinxawesome-theme/releases/tag/5.2.0

docs/source/conf.py

@@ -16,7 +16,6 @@
 author = 'The PHP Group'
 extensions = [
     'sphinx_design',
-    'sphinxawesome_theme.highlighting',
 ]
 templates_path = ['_templates']
 html_theme = 'sphinxawesome_theme'