deps: update dependency com.google.protobuf:protobuf-bom to v3.15.3 (#391)

Author: renovate-bot

java-core/pom.xml

@@ -159,7 +159,7 @@
     <google.api.version>1.31.2</google.api.version>
     <google.http.version>1.38.1</google.http.version>
     <grpc.version>1.34.1</grpc.version>
-    <protobuf.version>3.15.2</protobuf.version>
+    <protobuf.version>3.15.3</protobuf.version>
     <opencensus.version>0.24.0</opencensus.version>
     <annotations-api.version>1.3.2</annotations-api.version>
     <guava.version>30.1-android</guava.version><!-- We are currently using the *-android version to support JDK7. -->