doc: lock down jinja2 in requirements.txt

Problem: readthedocs CI build is failing with

  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/home/docs/checkouts/readthedocs.org/user_builds/flux-core/envs/5492/lib/python3.11/site-packages/sphinx/__main__.py", line 13, in <module>
    from sphinx.cmd.build import main
  File "/home/docs/checkouts/readthedocs.org/user_builds/flux-core/envs/5492/lib/python3.11/site-packages/sphinx/cmd/build.py", line 25, in <module>
    from sphinx.application import Sphinx
  File "/home/docs/checkouts/readthedocs.org/user_builds/flux-core/envs/5492/lib/python3.11/site-packages/sphinx/application.py", line 43, in <module>
    from sphinx.registry import SphinxComponentRegistry
  File "/home/docs/checkouts/readthedocs.org/user_builds/flux-core/envs/5492/lib/python3.11/site-packages/sphinx/registry.py", line 24, in <module>
    from sphinx.builders import Builder
  File "/home/docs/checkouts/readthedocs.org/user_builds/flux-core/envs/5492/lib/python3.11/site-packages/sphinx/builders/__init__.py", line 26, in <module>
    from sphinx.util import import_object, logging, progress_message, rst, status_iterator
  File "/home/docs/checkouts/readthedocs.org/user_builds/flux-core/envs/5492/lib/python3.11/site-packages/sphinx/util/rst.py", line 21, in <module>
    from jinja2 import Environment, environmentfilter
ImportError: cannot import name 'environmentfilter' from 'jinja2' (/home/docs/checkouts/readthedocs.org/user_builds/flux-core/envs/5492/lib/python3.11/site-packages/jinja2/__init__.py)

See readthedocs/readthedocs.org#9038.  Looks like this is a problem
with newer "jinja".  Do what one of the people in that issue did.

doc/requirements.txt

@@ -2,3 +2,4 @@ sphinx==3.4.3
 sphinx-rtd-theme>=0.5.2
 docutils>=0.14,<0.18
 urllib3<2
+jinja2<3.1