Migrate to use ascopes/protobuf-maven-plugin

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

Start with the repository-wide search for org.xolstice.maven.plugins linked in the issue, then compare each usage with ascopes/protobuf-maven-plugin's documentation. Update every old-plugin reference and verify that the google-cloud-java build completes successfully with the replacement.

索引モデルが issue の本文から書いたものです。