doc: drop sphinx < 6.0.0

Problem: The requirements-doc.txt file for flux-core documentation
requires sphinx < 6.0.0, but we would like to upgrade sphinx to the
latest version.

Drop the sphinx < 6.0.0 requirement.

Author: cmoussa1

scripts/requirements-doc.txt

@@ -1,4 +1,3 @@
-sphinx<6.0.0
 sphinx-rtd-theme>=0.5.2
 docutils>=0.14,<0.18
 urllib3<2