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.

Update protobuf version

Update protobuf version

Update protobuf version