gcp-py-instance-nginx: migrate off the deprecated gce-container-declaration metadata #3055

gcp-py-instance-nginx/__main__.py puts a gce-container-declaration key in the instance metadata. pulumi-gcp v10 deprecates this; it still deploys but warns (see the v10 migration guide). Move the example to a supported approach, such as a cloud-init startup script that runs the container, or Cloud Run.

Part of pulumi/pulumi-gcp#3950