[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.

---

• Update deprecated on_event with lifespan event
• Update tutorial text
• Update tutorial line numbers

[github-actions]

📝 Docs preview for commit 79462c5 at: https://32aff99e.sqlmodel.pages.dev