Chore: route uptime check through FrontDoor (#281)

terraform/uptime.tf

@@ -6,7 +6,7 @@ module "healthcheck" {
   application_insights = azurerm_application_insights.main
   # not strictly necessary to include the environment name, but helps to make the alerts more clear
   name = "mst-courtesy-cards-eligibility-server-${local.env_name}-healthcheck"
-  url  = "https://${azurerm_linux_web_app.main.default_hostname}/healthcheck"
+  url  = "https://${azurerm_cdn_frontdoor_endpoint.main.host_name}/healthcheck"
 }
 
 # ignore when app restarts as data is being reloaded