Update version to 3.19.0-rc2 and fix changelog about Java 7 (#9116)
Update version to 3.19.0-rc2 and fix changelog about Java 7 (#9116)

* Update protobuf version

* Update CHANGES.txt to reflect that we're not yet dropping Java 7
protocolbuffers/protobuf-javascript,protocolbuffers/protobuf-javascript,protocolbuffers/protobuf-javascript,protocolbuffers/protobuf-javascript