deps: update protobuf.version to v3.12.2 (#320)

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>3.12.1</protobuf.version>
+    <protobuf.version>3.12.2</protobuf.version>
     <threeten.version>1.4.4</threeten.version>
   </properties>