tests: revert bumping jinja2 in flask 1.0 tests (#2037)

Unfortunately flask 1.0 does not work with jinja 3.1.x:

venv/lib/python3.10/site-packages/flask/__init__.py:19: in <module>
    from jinja2 import Markup, escape
E   ImportError: cannot import name 'Markup' from 'jinja2'

tests/requirements/reqs-flask-1.0.txt

@@ -1,4 +1,4 @@
-jinja2<3.2.0
+jinja2<3.1.0
 Werkzeug<2.1.0
 Flask>=1.0,<1.1
 MarkupSafe<2.1