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 deprecation warnings, specifically the one mentioned above and reported in the Warnings Report.

Changed logger.warn to logger.warning in the following files:

    lms/djangoapps/experiments/utils.py

    lms/djangoapps/lti_provider/outcomes.py

    lms/djangoapps/courseware/module_render.py

This warning occurs due to deprecation in python 3.4+: https://docs.python.org/3.5/library/logging.html#logging.Logger.warning