docs: Fix furo requirement after recently breaking docs

PR 4736 bumped the version of sphinx we use on readthedocs, and that
was incompatible with the old version of furo we specified.

Signed-off-by: Larry Gritz <lg@larrygritz.com>

Author: lgritz

src/doc/requirements.txt

@@ -1,4 +1,4 @@
 sphinx >= 8.0
 breathe == 4.36.0
 sphinx-tabs
-furo==2022.6.21
+furo==2024.8.6