Ubuntu 18.04/16.04 still referenced in docker-rancher demo template

Both of these releases have exited standard support and require use of a Pro sku (or a relationship with Canonical) to continue receiving security updates. Deploying with the sku as-is poses an ongoing security risk.

https://github.com/Azure/azure-quickstart-templates/blob/3bee94e36dade8c5931629b2ad78d3c30ce38536/demos/docker-rancher/server.json#LL39C5-L39C5

Open demos/docker-rancher/server.json at the linked line and confirm the Ubuntu release reference and the repository’s supported-image conventions. Update the outdated reference to a supported release, then verify that the template no longer requests an end-of-support Ubuntu version.

Written by the indexing model from the issue text.