Replace postgresql dialect UUID field to backend agnostic UUID sqlalchemy field

[rafsaf]

https://github.com/rafsaf/minimal-fastapi-postgres-template/blob/32101f2a899e35fc3959a15a2144e3a1fc37ac73/%7B%7Bcookiecutter.project_name%7D%7D/app/models.py#L19C1-L19C48

• UUID field should be replaced by backend agnostic field: https://docs.sqlalchemy.org/en/20/core/type_basics.html#sqlalchemy.types.Uuid
• rethink if as_uuid=False was a good choice
• regenerate migration file

[rafsaf]

done https://github.com/rafsaf/minimal-fastapi-postgres-template/pull/44