Filter unpublished events in EventListAPIView

[bilibilistack]

Change Summary

#131

Add filter logic to display results of Eventslist, only events after the publicationDate could be fetched from the API and displayed on the page.

• Updated the queryset in get_queryset (in server/game_dev/views.py) to filter events by publicationDate, showing only those that have already been published.

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Related issue

• Resolve Upcoming events list would leak unpublished events #131