🗑️ Deprecate const and unique_items in Field

[YuriiMotov]

const and unique_items parameters in Field are deprecated in Pydantic V2 (const, unique_items):

    const = extra.pop('const', None)  # type: ignore
    if const is not None:
        raise PydanticUserError('`const` is removed, use `Literal` instead', code='removed-kwargs')
...

    unique_items = extra.pop('unique_items', None)  # type: ignore
    if unique_items is not None:
        raise PydanticUserError(
            (
                '`unique_items` is removed, use `Set` instead'
                '(this feature is discussed in https://github.com/pydantic/pydantic-core/issues/296)'
            ),
            code='removed-kwargs',
        )

This PR reflects this change.

---

These parameters don't work (just ignored) on current SQLModel version. And they never worked with SQLMOdel + Pydantic V2.
So, not sure we need to mark this PR as breaking, but I did it it worked in some previous versions with Pydantic V1

[tiangolo]

Thank you! 🙌

Yes, I think it's a good idea to just raise the error, if this has never worked with Pydantic v2, better to not have it around and just show the error.