Do not use class method for model validator for pydantic 2.12

[NeejWeej]

According to this PR: pydantic/pydantic#11957

model_validators shouldn't be class methods.

Follows the example here:
https://github.com/pydantic/pydantic/blob/v2.12.0/tests/test_model_validator.py#L85

from the pydantic test suite to have a model validator that takes a ValidationInfo parameter

(as-of time of writing)

def test_model_validator_after() -> None:
    class Model(BaseModel):
        x: int
        y: int

        @model_validator(mode='after')
        def val_model(self, info: ValidationInfo) -> Model:
            assert not info.context
            self.x += 1
            self.y += 1
            return self

    assert Model(x=1, y=2).model_dump() == {'x': 2, 'y': 3}
    assert Model.model_validate(Model(x=1, y=2)).model_dump() == {'x': 3, 'y': 4}