Updated version to 3.15.8 (#8463)
Updated version to 3.15.8 (#8463)

* Update protobuf version

* Updated CHANGES.txt and package.xml for 3.15.8
protocolbuffers/protobuf-javascript,protocolbuffers/protobuf-javascript,protocolbuffers/protobuf-javascript,protocolbuffers/protobuf-javascript