[WIP] Update `on_event` with lifespan event handler in docs

[ppommer]

Taking care of

DeprecationWarning: 
          on_event is deprecated, use lifespan event handlers instead.
  
          Read more about it in the
          [FastAPI docs for Lifespan Events](https://fastapi.tiangolo.com/advanced/events/).

in the FastAPI and Pydantic Tutorial.

Deprecation is noted here and here.

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• [x] Update deprecated on_event with lifespan event
• [x] Update tutorial text
• [ ] Update tutorial line numbers

[github-actions[bot]]

📝 Docs preview for commit 79462c54dc090c0202f4dfb526e65724de4b831b at: https://32aff99e.sqlmodel.pages.dev