Unpin pydantic version

[oyvindeide]

We had to pin pydantic after 2.10 was release due to the following error:

E       pydantic.errors.PydanticUserError: `ErtConfig` is not fully defined; you should define `List`, then call `pydantic.dataclasses.rebuild_dataclass(ErtConfig)`.
E       
E       For further information visit https://errors.pydantic.dev/2.10/u/class-not-fully-defined

/Users/OYVEID/projects/ert/ert/src/ert/config/ert_config.py:525: PydanticUserError

to reproduce upgrade pydantic and run for example:
tests/ert/unit_tests/config/test_ert_config.py