I wanted to make a byte-sized contribution but there were no Jira tickets so we decided, thanks to a conversation with @jmbowman through the Open Edx Community #incr (Slack) channel, to collaborate in the elimination of warnings listed in the Warnings Report at https://build.testeng.edx.org/job/edx-platform-python-pipeline-master/warning_5freport_5fall_2ehtml/

This PR contributes to the elimination of RemovedInDjango30Warnings, specifically the one mentioned above and reported in the Warnings Report

Changed cookie_date to http_date in the following file:

    openedx/core/djangoapps/user_authn/cookies.py

This warning occurs due to deprecation since Django 2.1: https://docs.djangoproject.com/en/2.2/ref/utils/#django.utils.http.cookie_date