deps: update dependency com.google.protobuf:protobuf-java to v3.12.2 (#191)

Author: renovate-bot

pom.xml

@@ -69,7 +69,7 @@
     <google.iam.version>0.13.0</google.iam.version>
     <gax.version>1.56.0</gax.version>
     <grpc.version>1.30.0</grpc.version>
-    <protobuf.version>3.12.0</protobuf.version>
+    <protobuf.version>3.12.2</protobuf.version>
     <junit.version>4.13</junit.version>
     <guava.version>29.0-android</guava.version>
     <threeten.version>1.4.4</threeten.version>