Update protobuf version

Update protobuf version

Update protobuf version

Update protobuf version

Update protobuf version

Update protobuf version

Update protobuf version

Added PHP changes for 3.15.0 into package.xml

Added PHP changes for 3.15.0 into package.xml

Removed protoc-artifacts/target directory

I committed this directory by mistake and these build artifacts should
not be checked in.