Ensure 'always_on' feature is enabled for Azure App Service

Azure App Service that doesn't enable 'always on' feature may lead to low resilience.

In Azure Console -

1. Open the Azure Portal and go to App Services.
2. Select the App service you wish to edit.
3. Select Configuration under Settings, then the General settings tab.
4. Always on to On.
5. Save.

In Terraform -

1. In the azurerm_app_service resource, create a site_config block.
2. Set the site_config.always_on field to true.

References:
https://learn.microsoft.com/en-us/azure/app-service/configure-common?tabs=portal
https://registry.terraform.io/providers/hashicorp/azurerm/latest/docs/resources/app_service#site_config