The [xolstice's protobuf-maven-plugin](https://github.com/xolstice/protobuf-maven-plugin) is deprecated and no longer being maintained. Recommendation is to use https://github.com/ascopes/protobuf-maven-plugin. In google-cloud-java, there are these references to the old plugin: https://github.com/search?q=repo%3Agoogleapis%2Fgoogle-cloud-java%20org.xolstice.maven.plugins&type=code
The xolstice's protobuf-maven-plugin is deprecated and no longer being maintained.
Recommendation is to use https://github.com/ascopes/protobuf-maven-plugin.
In google-cloud-java, there are these references to the old plugin: https://github.com/search?q=repo%3Agoogleapis%2Fgoogle-cloud-java%20org.xolstice.maven.plugins&type=code