fix: pin openedx-events to version with eventsmetadata serialization

[rgraber]

Pin openedx-events to 8.5.0, which has EventsMetadata serialization.

Merge checklist: Check off if complete or not applicable:

• [ ] Version bumped
• [ ] Changelog record added
• [ ] Documentation updated (not only docstrings)
• [ ] Commits are squashed
• [ ] Noted any: Concerns, dependencies, deadlines, tickets, testing instructions