Migrate Azure Spring Apps backend to Azure Container Apps

Azure Spring Apps is retired and this template needs to migrate its Java Spring Boot backend from Azure Spring Apps to Azure Container Apps.

This repository will be temporarily removed from the [awesome-azd gallery](https://azure.github.io/awesome-azd/) while the migration is pending. After the migration is complete, feel free to submit a request to add the template back to the gallery.

Related references:
- Azure Spring Apps retirement guidance: https://aka.ms/asaretirement
- Gallery cleanup: Azure/awesome-azd#955

cc @pmalarme as you are one of the authors

The issue names no files or tests. Start by inventorying the template's Azure Spring Apps and Java Spring Boot deployment configuration, then read the linked Azure Spring Apps retirement guidance and compare the required Azure Container Apps setup. Done means the backend runs on Azure Container Apps and the template is ready to return to the gallery.

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