Switch from deprecated server on_startup/shutdown hooks to lifespan

[zanieb]

First check

• I am a contributor to the Prefect codebase

Description

Starlette has deprecated the on_startup and on_shutdown hooks we are using for managing services. We should use the new interface.

See

• https://www.starlette.io/lifespan/
• Further develop startup and shutdown events tiangolo/fastapi#617 (comment)
• Future of on_startup/on_shutdown and lifespan encode/starlette#2067
• Alternative lifespan state api encode/starlette#2065

Impact

This will make us compatible with future versions of Starlette.

Additional context

We should remove the ignore added in #8785