Update docs theme and remove docs env pins

Author: jacobtomlinson

docs/requirements.txt

@@ -3,15 +3,8 @@ tornado
 toolz
 cloudpickle
 git+https://github.com/dask/dask
-sphinx
-dask-sphinx-theme>=3.0.0
-# FIXME: `sphinxcontrib-*` pins are a workaround until we have sphinx>=5.
-#        See https://github.com/dask/dask-sphinx-theme/issues/68.
-sphinxcontrib-applehelp>=1.0.0,<1.0.7
-sphinxcontrib-devhelp>=1.0.0,<1.0.6
-sphinxcontrib-htmlhelp>=2.0.0,<2.0.5
-sphinxcontrib-serializinghtml>=1.1.0,<1.1.10
-sphinxcontrib-qthelp>=1.0.0,<1.0.7
+sphinx>=8.0.0
+dask-sphinx-theme>=4.0.0
 sphinx-click
 memray
 sphinx_design