Remove Django and Python versions from the documentation

Once again, they fell out of date. Refer users to the supported version
Django and Python. Also, PyPI and other tools can read that metadata
from `pyproject.toml`.

docs/index.rst

@@ -12,7 +12,9 @@ groups, and permissions.
 * Repository: https://github.com/django-auth-ldap/django-auth-ldap
 * License: BSD 2-Clause
 
-This version is supported on Python 3.8+; and Django 2.2+. It requires
+This version is supported all supported version of `Python
+<https://www.python.org/downloads/>`_ and `Django
+<https://www.djangoproject.com/download/#supported-versions>`_. It requires
 `python-ldap`_ >= 3.1.
 
 .. toctree::