make container apps a single instance resource

[kewalaka]

removing the possibility of creating multiple container apps within the module, as per AVM guidance;

https://azure.github.io/Azure-Verified-Modules/specs/shared/#id-rmfr1---category-composition---single-resource-only

ID: RMFR1 - Category: Composition - Single Resource Only

A resource module MUST only deploy a single instance of the primary resource, e.g., one virtual machine per instance.

Multiple instances of the module MUST be used to scale out.