🗑️ 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.

I ended up just emitting a warning (not error) in case someone use these parameters. Should we also raise error as Pydantic does?

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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