📝 Update documentation for deprecated on_startup and on_shutdown params

[heysaeid]

Description: Update documentation for deprecated on_startup and on_shutdown parameters.

Details: This PR addresses the deprecation of the on_startup and on_shutdown parameters in the annotated type within FastAPI and APIRouter. The documentation for these parameters has been updated to reflect their deprecation and to recommend using the lifespan parameter instead, as per the FastAPI documentation guidelines.

Changes Made:

• Updated the documentation within the annotated type for the deprecated on_startup and on_shutdown parameters.
• Added information highlighting the deprecation and recommended usage of the lifespan parameter.

Branch Name: docs/deprecate-on-startup-shutdown-params

[heysaeid]

Hi @alejsdev 👋 May I know what the P4 label means and when PRs that have this label will be reviewed?