fix: use correct event type for forum events

[mariajgrimaldi]

Description

This PR fixes the forum IDs so:

1. They match each event definition.
2. They're unique.

This change shouldn't be considered a breaking change since these events are not being sent to the event bus yet, so the event_type is not actively used by consumers.

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[openedx-webhooks]

Thanks for the pull request, @mariajgrimaldi!

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