Upgrade .circleci/config.yml

[cclauss]

Perform various upgrades as prompted by the CircleCI extension for VSCode.

• https://marketplace.visualstudio.com/items?itemName=circleci.circleci

~Should Node.js be upgraded as the current version is v22.9.0?~ --> https://github.com/readthedocs/readthedocs.org/pull/11667#pullrequestreview-2365946664

[cclauss]

Not ready for Python 3.12?

• https://circleci.com/gh/readthedocs/readthedocs.org/28368 and
• https://circleci.com/gh/readthedocs/readthedocs.org/28366

    @property
    def backend(self):
        """Current backend instance."""
        try:
>           return self._local.backend
E           AttributeError: '_thread._local' object has no attribute 'backend'

.tox/ext-theme/lib/python3.12/site-packages/celery/app/base.py:1250: AttributeError

[ericholscher]

Sorry we missed this. It looks like this is conflicting now, and not a huge priority, so going to close this for now. Feel free to open a new one if this is still something that's useful.