Update px-mongo demo to use ghcr.io mirrored images

[ddelnano]

Summary: Update px-mongo demo to use ghcr.io mirrored images

Relevant Issues: N/A

Type of change: /kind infra

Test Plan: Deployed the demo by building and serving the px-mongo tar.gz locally

$ bazel build //demos:px-mongo
$ cp bazel-bin/demos/px-mongo.tar.gz demos/ && cd demos && python -m http.server
$ px demo deploy  --artifacts http://localhost:8000 px-mongo

Changelog Message: Moved hosting of px-mongo demo container images from gcr.io to ghcr.io