Update examples to GA API

examples/python/helloworld/greeter_client.py

examples/python/helloworld/greeter_server.py

examples/python/helloworld/helloworld_pb2.py

examples/python/helloworld/run_codegen.py

examples/python/route_guide/route_guide_client.py

examples/python/route_guide/route_guide_pb2.py

examples/python/route_guide/route_guide_resources.py

examples/python/route_guide/route_guide_server.py

examples/python/route_guide/run_codegen.py