Updated version to 3.15.8 (protocolbuffers#8463)

* Update protobuf version

* Updated CHANGES.txt and package.xml for 3.15.8

Update protobuf version (protocolbuffers#8448)

Update protobuf version

Update protobuf version

Update protobuf version

Update protobuf version

Update protobuf version

Update protobuf version

Update protobuf version

Update protobuf version