replace deprecated fastapi example, set min fastapi version

fix:

/home/yang/nomad/nomad-distro-dev/packages/nomad-FAIR/nomad/app/v1/models/models.py:380: FastAPIDeprecationWarning: `example` has been deprecated, please use `examples` instead

https://fastapi.tiangolo.com/tutorial/schema-extra-example/?h=body#summary

In short, upgrade to FastAPI 0.99.0 or above, and things are much simpler, consistent, and intuitive, and you don't have to know all these historic details. 😎