DEPR: read_gbq, DataFrame.to_gbq

[mroeschke]

1. These functions are a 1 line call to https://googleapis.dev/python/pandas-gbq/latest/api.html#pandas_gbq.read_gbq and https://googleapis.dev/python/pandas-gbq/latest/api.html#pandas_gbq.to_gbq, so pandas' signatures will always lag behind the full functionality of the underlying library call.
2. We don't have any tests for this functionality nor have the capability to test this
3. It's caused some testing headaches in the past CI: Pin pandas-gbq above 0.15 to get rid of warnings on 2.0.x #53693 CI: Bump pandas-gbq to one that doesn't pin pyarrow #54200

I think it would be best if we direct user to just use the underlying library in our ecosystem/IO docs

[jbrockmendel]

We don't have any tests for this functionality nor have the capability to test this

i find this reason compelling