deps: update dependency com.google.protobuf:protobuf-bom to v3.12.2

Author: renovate-bot

pom.xml

@@ -70,7 +70,7 @@
  <google.http-client.version>1.35.0</google.http-client.version>
  <gax.version>1.56.0</gax.version>
  <grpc.version>1.29.0</grpc.version>
- <protobuf.version>3.12.1</protobuf.version>
+ <protobuf.version>3.12.2</protobuf.version>
  <junit.version>4.13</junit.version>
  <easymock.version>3.6</easymock.version>
  <guava.version>29.0-android</guava.version>