Update module github.com/googleapis/gax-go/v2 to v2.12.3

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:pipeline:curl -sS "https://raw.githubusercontent.com/bazelbuild/continuous-integration/master/buildkite/bazelci.py?$(date +%s)" -o bazelci.py && python3.6 bazelci.py project_pipeline | tee /dev/tty | buildkite-agent pipeline upload
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Buildifier
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:ubuntu: Ubuntu 16.04 LTS (OpenJDK 8, gcc 5.4.0)curl -sS https://raw.githubusercontent.com/bazelbuild/continuous-integration/master/buildkite/bazelci.py?1710458438 -o bazelci.py && python3.6 bazelci.py runner --task=presubmit
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Total Job Run Time: 41s