classpath conflicts due to transitive dependencies of spanner and mysql drivers

[timveil]

both the spanner jdbc driver and the mysql driver specify com.google.protobuf:protobuf-java as a dependency. Mysql version 8.0.27 depends on version 3.11.4 while spanner version 2.5.5 depends on version 3.19.1. When running spanner tests, the mysql required version is loaded causing NoSuchMethod exceptions at runtime.