Use '.. confval::` in Sphinx 7.4+

[dgw]

When documenting settings in the core section or elsewhere, we can use newer Sphinx functionality once it's supported by the Furo theme.

See: https://www.sphinx-doc.org/en/master/usage/domains/standard.html#directive-confval

See: pradyunsg/furo#901 (this can be become a "blocked by" relationship if/when it's turned into an issue)