doc(pubsub): update Doxygen version numbers (#5041)
doc(pubsub): update Doxygen version numbers (#5041)

We need to publish the docs to https://googleapis.dev with the current
version or they do not show up in the "latest" link. Still mark the
current code as "Beta" so users know what to expect.
googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp