Bump protobuf-java-util in /data-prepper-plugins/otel-trace-source

Bumps [protobuf-java-util](https://github.com/protocolbuffers/protobuf) from 3.15.5 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@v3.15.5...v3.15.6)

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

data-prepper-plugins/otel-trace-source/build.gradle

@@ -8,7 +8,7 @@ dependencies {
     compile 'commons-codec:commons-codec:1.15'
     testCompile project(':data-prepper-api').sourceSets.test.output
     implementation "io.opentelemetry:opentelemetry-proto:${versionMap.opentelemetry_proto}"
-    implementation 'com.google.protobuf:protobuf-java-util:3.15.5'
+    implementation 'com.google.protobuf:protobuf-java-util:3.15.6'
     implementation "com.linecorp.armeria:armeria:1.5.0"
     implementation "com.linecorp.armeria:armeria-grpc:1.5.0"
     implementation "com.fasterxml.jackson.core:jackson-databind:2.12.2"