Pin google-cloud-bigquery to < 3.21.0

[pankajastro]

related-to: https://github.com/apache/airflow/issues/39541

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[potiuk]

FYI. Whenever we put upper-binding, we should always make a comment and add link to the issue that should remain open - describing the problem. Ideally maybe even opening it in the offending library. Not sure if that's the case here (liley @VladaZakharova and her team should take a look there) - also hopefully system tests dashboard that @ahidalgob works on at google should be able to detect those cases.