Pin minimum version of furo to 2024.8.6

[weiji14]

Try to fix incompatibility with sphinx=8.0 causing AttributeError: _CascadingStyleSheet is immutable with old versions of furo.

Full traceback error from Readthedocs:

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/cupy-xarray/conda/63/lib/python3.10/site-packages/sphinx/events.py", line 100, in emit
    results.append(listener.handler(self.app, *args))
  File "/home/docs/checkouts/readthedocs.org/user_builds/cupy-xarray/conda/63/lib/python3.10/site-packages/furo/__init__.py", line 158, in _html_page_context
    _add_asset_hashes(
  File "/home/docs/checkouts/readthedocs.org/user_builds/cupy-xarray/conda/63/lib/python3.10/site-packages/furo/__init__.py", line 138, in _add_asset_hashes
    static[index].filename = _asset_hash(asset)  # type: ignore
  File "/home/docs/checkouts/readthedocs.org/user_builds/cupy-xarray/conda/63/lib/python3.10/site-packages/sphinx/builders/html/_assets.py", line 54, in __setattr__
    raise AttributeError(msg)
AttributeError: _CascadingStyleSheet is immutable

References:

• Furo is broken on Sphinx 7.2.0 and 7.2.1 pradyunsg/furo#693
• [CI] Update conda lock files MetOffice/CSET#783 (review)