Bump protobuf-java from 3.15.3 to 3.15.6

Bumps [protobuf-java](https://github.com/protocolbuffers/protobuf) from 3.15.3 to 3.15.6.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.15.3...v3.15.6)

Signed-off-by: dependabot[bot] <support@github.com>

pom.xml

@@ -137,7 +137,7 @@
     <dependency>
       <groupId>com.google.protobuf</groupId>
       <artifactId>protobuf-java</artifactId>
-      <version>3.15.3</version>
+      <version>3.15.6</version>
     </dependency>
 
     <!-- Spark Core pulls in too old of a version -->