feat: take exec ed course data from course run instead of additional_metadata attempt 3

[marlonkeating]

Description

Updates references for Exec Ed courses originally contained in the additional_metadata field, to now be taken from course runs. Fixing datetime parsing issue that was found in earlier attempt https://github.com/openedx/enterprise-catalog/pull/912, by incorporating fixes made in https://github.com/openedx/enterprise-catalog/pull/958

Ticket Link

ENT-8248: update custom logic for non-ocm courses in enterprise-catalog

Outstanding Questions

• [ ] Do we still need to process Exec Ed courses differently from OCM courses?

Post-review

• [ ] Squash commits into discrete sets of changes
• [ ] Ensure that once the changes have been deployed to stage, prod is manually deployed