langchain: update declarative_base import (#20056)
**Description**: The ``declarative_base()`` function is now available as
sqlalchemy.orm.declarative_base(). (depreca ted since: 2.0) (Background
on SQLAlchemy 2.0 at: https://sqlalche.me/e/b8d9)
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Co-authored-by: Isaac Francisco <78627776+isahers1@users.noreply.github.com>
Co-authored-by: isaac hershenson <ihershenson@hmc.edu>
