'Update the pin on prefect version'

[urimandujano]

As part of our work adding support for Pydantic 2, we removed its pin in
prefect's requirements.txt. This means that it's possible to have
prefect, pydantic>=2, and any version of this collection installed. But,
the collection(s) only support pydantic>=2 in their latest versions. This
PR adds a pin on the collection's requirements.txt to make sure that it is
only installed with the correct version prefect that supports pydantic>=2

[chrisguidry]

Ah yes, I think the unit tests were failing so I was never able to get this one updated, fell through the cracks :doh:

[urimandujano]

I finally got the tests here passing. The upshot is that two default tests (that we inherit from prefect) were failing so I used some good ol' OOP to customize them:

1. skip a test that will fail on Windows
2. increase a timeout on a heavy test (ray_task_runner seem to be particularly slow on Windows)