deps: update protobuf.version to v3.11.3 (#187)

Author: renovate-bot

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

@@ -83,7 +83,7 @@
  <grpc.version>1.27.0</grpc.version>
  <guava.version>28.2-android</guava.version>
  <opencensus.version>0.24.0</opencensus.version>
- <protobuf.version>3.11.1</protobuf.version>
+ <protobuf.version>3.11.3</protobuf.version>
  <threeten.version>1.4.1</threeten.version>
  </properties>