Move `googlecloudplatform/bq-schema-api` to sync semver releases

[unmultimedio]

The source repository https://github.com/GoogleCloudPlatform/protoc-gen-bq-schema does SemVer releases, it can be moved to sync them instead of individual git commits.

We need to wait for the next release to avoid pushing breaking changes to the BSR, since we're currently syncing the main branch, which already includes some new proto fields after the latest release.