FastAPI: add dependabot to template dependencies

[h3rmanj]

Template dependencies should be managed by dependabot (if possible).

[JonasKs]

In poetry you say poetry update, which will soft-bump anything that's not a major release on install. So in general, users will never have a minor or patch that isn't bumped.
With that said, there is no reason to not bump pyproject.toml, so I'll fix this. 😊