♻️ Ignore `on_event is deprecated` deprecation warnings, fix `ValidationError` import warning

[YuriiMotov]

Currently we have a lot of deprecation warnings because we use fastapi.on_event in docs examples. These warnings are useless and make it hard to read log of tests. I suggest we just ignore "on_event is deprecated" warnings globally.

I also fixed import of ValidationError which also produced a deprecation warning.

With Pydantic V2 we still have 1 type of warnings:

SAWarning: Object of type <HeroTeamLink> not in session, add operation along 'Hero.team_links' will not proceed (This warning originated from the Session 'autoflush' process, which was invoked automatically in response to a user-initiated operation. Consider using ``no_autoflush`` context manager if this warning happened while initializing objects.)

Not sure how to fix this one..

See log

tests/test_tutorial/test_many_to_many/test_tutorial003.py::test_tutorial
tests/test_tutorial/test_many_to_many/test_tutorial003_py310.py::test_tutorial
tests/test_tutorial/test_many_to_many/test_tutorial003_py39.py::test_tutorial
  /home/yurii/code/sqlmodel_forks/motov/sqlmodel/sqlmodel/orm/session.py:144: SAWarning: Object of type <HeroTeamLink> not in session, add operation along 'Hero.team_links' will not proceed (This warning originated from the Session 'autoflush' process, which was invoked automatically in response to a user-initiated operation. Consider using ``no_autoflush`` context manager if this warning happened while initializing objects.)
    return super().execute(

tests/test_tutorial/test_many_to_many/test_tutorial003.py::test_tutorial
tests/test_tutorial/test_many_to_many/test_tutorial003_py310.py::test_tutorial
tests/test_tutorial/test_many_to_many/test_tutorial003_py39.py::test_tutorial
  /home/yurii/code/sqlmodel_forks/motov/sqlmodel/sqlmodel/orm/session.py:144: SAWarning: Object of type <HeroTeamLink> not in session, add operation along 'Team.hero_links' will not proceed (This warning originated from the Session 'autoflush' process, which was invoked automatically in response to a user-initiated operation. Consider using ``no_autoflush`` context manager if this warning happened while initializing objects.)
    return super().execute(

With Pydantic V1 we still have a lot of warnings, but they are not easily fixable. Anyway, with this PR we will have less warnings and it will be easier to review test logs

[github-actions[bot]]

📝 Docs preview

Last commit 7c8a3308d1ccf4c4e5c6b97ac44471532e8f4bec at: https://4709c771.sqlmodel.pages.dev

[YuriiMotov]

Maybe I've forgotten/missed some previous discussion about this, but why don't we update the usage of on_event, e.g. like #958 attempts to do?

#958 uses blocking function inside async lifespan function. That's not a good practice..

@asynccontextmanager
async def lifespan(app: FastAPI):
    create_db_and_tables()
    yield

We can update it to run it in the threadpool, but it will make the code example more complicated..