doc: remove jinja2<3.1 requirement

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

Remove the jinja2 version requirement from requirements.txt.

Author: cmoussa1

requirements.txt

@@ -4,7 +4,6 @@ sphinxcontrib-spelling
 sphinx-rtd-theme>=0.5.2
 sphinx-copybutton
 docutils
-Jinja2<3.1
 markupsafe==2.0.1
 sphinx_immaterial
 pydantic