Migrate from admin auth to Managed Identity for Container Registry

[aaronpowell]

The Azure Container Registry that is deployed as part of the Bicep templates (which is in the default container-apps.bicep file) uses admin auth to authenticate rather than a managed identity.

This results in an S360 violation when deployed, as we should have admin auth disabled and instead use secret-less auth models.

Might require updated templates (see #325) before it can be done.