Re-declare BigQuery as GA

[garrettjonesgoogle]

We tried to bump BigQuery to GA in #2982 , but it got erased in the version bump of the release at https://github.com/GoogleCloudPlatform/google-cloud-java/pull/3004/files#diff-9fa081a8634fe9e631cd85762bb9ceb9 . The thing we need to do differently is to change the released-version version in the versions.txt file for bigquery instead of only current-version, since the version bump script bases the bump on released-version. Like last time, we shouldn't update the README files until release.

[pongad]

Does this boil down to changing versions.txt file but not running the update script (yet)? When we release, the version should get bumped properly then propagated?

[garrettjonesgoogle]

Hmm yes, that's what we should do.