fix: use model_copy instead of copy for pydantic 2 compatibility

[neubi4]

In pydantic 2, copy of a model is deprecated, we should use model_copy instead.

Migration Guide: https://docs.pydantic.dev/2.0/migration/#migration-guide

[rafalkrupinski]

For this it would be better to put in dependencies the maximum fastapi version that works with pydantic 1., and only apply this PR with the fastapi version that works with pydantic 2..

It's a breaking change, but since fastapi is on 0.* releases, anything goes

[wisdomtooth546]

There are other instances where this copy() appears, such as InMemmoryBackend(), in the code so might want to update them as well.