deps: update protobuf.version to v3.12.1 (#308)

Author: renovate-bot

google-cloud-bigtable-deps-bom/pom.xml

@@ -83,7 +83,7 @@
  <grpc.version>1.29.0</grpc.version>
  <guava.version>29.0-android</guava.version>
  <opencensus.version>0.26.0</opencensus.version>
- <protobuf.version>3.11.4</protobuf.version>
+ <protobuf.version>3.12.1</protobuf.version>
  <threeten.version>1.4.4</threeten.version>
  </properties>