Fix code scanning alert - Virtual network (VNET) subnets should have Network Security Groups (NSGs) assigned.

[tonybaloney]

Tracking issue for:

• https://github.com/Azure-Samples/azure-search-openai-demo/security/code-scanning/53

[pamelafox]

The latest change adds vnets to container apps and adds the NSGs recommended by the Azure documentation. I think that's as good as we can get, given the difficulty of setting up NSGs with App Service and Oryx, and App Service is no longer the default option.

#2322