[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

[github-actions]

This pull request has a merge conflict that needs to be resolved.

[tiangolo]

Thanks for the interest and work on this! ☕

I want to implement a big refactor and some features in how to handle these things, I'm not fully happy with the existing interface.

I'll do that first, and then update the docs to reflect the recommended way to do things. I don't want to update the docs yet and suggest doing something that later I will suggest doing in a different way.

So for now, I will close this one. Thank you! 🍰