deps: update dependency com.google.protobuf:protobuf-java to v3.12.0 (#188)

Author: renovate-bot

pom.xml

@@ -68,7 +68,7 @@
     <google.common-protos.version>1.18.0</google.common-protos.version>
     <gax.version>1.56.0</gax.version>
     <grpc.version>1.29.0</grpc.version>
-    <protobuf.version>3.11.4</protobuf.version>
+    <protobuf.version>3.12.0</protobuf.version>
     <junit.version>4.13</junit.version>
     <easymock.version>4.2</easymock.version>
     <guava.version>29.0-android</guava.version>