Revert "Use new version of edx-when." (#28684)

Reverting because of errors seen in proctoring. The error from splunk:

 File "/edx/app/edxapp/venvs/edxapp/lib/python3.8/site-packages/edx_proctoring/views.py", line 590, in get
    data['onboarding_release_date'] = effective_start.isoformat()
AttributeError: 'NoneType' object has no attribute 'isoformat'

The effective date is coming from learning_sequences, which ultimately
comes from edx-when.