Migrate to Pydantic V2 API

[veenstrajelmer]

Is your feature request related to a problem? Please describe.
Dependency moved to Pydantic V2 in this PR: #588. However, this was done by keeping the V1 API calls. Pydantic V3 will be released this year. There will be no breaking changes this time, except for the fact that V1 API will be dropped (I think): pydantic/pydantic#10033. Either way, it seems to make sense to update to the V2 API asap. This will probably also make the HYDROLIB-core package more future proof.

Describe the solution you'd like
Update HYDROLIB-core to Pydantic V2 API.

Describe alternatives you've considered
None

Additional context
The V1 API now also shows a DeprecationWarning. It could well be this will break all code in the future, since it seems to happen on an import already, copied from the ubuntu Python 3.12 ci action:

=============================== warnings summary ===============================
.venv/lib/python3.12/site-packages/pydantic/v1/fields.py:437
  /home/runner/work/HYDROLIB-core/HYDROLIB-core/.venv/lib/python3.12/site-packages/pydantic/v1/fields.py:437: DeprecationWarning: datetime.datetime.utcnow() is deprecated and scheduled for removal in a future version. Use timezone-aware objects to represent datetimes in UTC: datetime.datetime.now(datetime.UTC).
    return smart_deepcopy(self.default) if self.default_factory is None else self.default_factory()