Migrate IContentDefinitionEventHandler to the new IContentDefinitionHandler

[MikeAlhayek]

Currently, the IContentDefinitionEventHandler interface introduces events that are triggered by the ContentDefinitionService. However, this service is primarily intended for use by controllers, and while it’s acceptable to retain it "if needed," it should not be responsible for triggering event handlers.

To improve consistency and ensure proper separation of concerns, all content definition events should be triggered by the ContentDefinitionManager, which is better suited for managing content definitions. Moving forward, the ContentDefinitionManager should be responsible for invoking events and continue using the IContentDefinitionHandler interface.

To streamline this process, the IContentDefinitionEventHandler interface and its associated events should be marked as obsolete. This approach ensures that event handling is centralized in the ContentDefinitionManager, leading to better maintainability and cleaner architecture.

[github-actions]

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