doc: remove jinja2<3.1 requirement

Problem: GitHub dependabot is reporting moderate security
vulnerabilities detected with jinja2 < 3.1.4, but flux-core has
jinja2<3.1.0 listed in its requirements-doc.txt file.

Remove the jinja2 version requirement from doc/requirements-doc.txt.

Author: cmoussa1

scripts/requirements-doc.txt

@@ -2,4 +2,3 @@ sphinx<6.0.0
 sphinx-rtd-theme>=0.5.2
 docutils>=0.14,<0.18
 urllib3<2
-jinja2<3.1.0