Bump protobuf-java from 3.15.3 to 3.15.6 (#181)

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>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

build.gradle

@@ -90,7 +90,7 @@ For more details, please see https://issues.apache.org/jira/browse/AURORA-1169
   ext.junitRev = '4.13.2'
   ext.logbackRev = '1.2.3'
   ext.nettyRev = '4.0.52.Final'
-  ext.protobufRev = '3.15.3'
+  ext.protobufRev = '3.15.6'
   ext.resteasyRev = '3.1.4.Final'
   ext.servletRev = '3.1.0'
   ext.slf4jRev = '1.7.30'