Needs update to support Pandas 3.0

Pandas was recently updated to 3.0.1, and pandas_gbq cannot be installed alongside it (it downgrades pandas to 2.x when using pip). pandas_gbq needs an update to support the 3.0 version of pandas.

[chalmerlowe]

Migration Notice: This library is moving to the google-cloud-python monorepo soon.

We are moving this issue to the new monorepo. Please continue this work in the new monorepo once the migration is complete!

[tswast]

To be fixed once we release a version of pandas-gbq with googleapis/python-bigquery-pandas#998