Alerting: Remove isDefault field from receivers (Alertmanager configuration) (#86605)

Alerting: Remove isDefault field from receivers in the Alertmanager configuration

pkg/services/ngalert/api/api_alertmanager_test.go

@@ -829,7 +829,6 @@ var validConfig = `{
 				"uid": "",
 				"name": "email receiver",
 				"type": "email",
-				"isDefault": true,
 				"settings": {
 					"addresses": "<example@email.com>"
 				}
@@ -937,7 +936,6 @@ var validConfigWithSecureSetting = `{
 				"uid": "",
 				"name": "email receiver",
 				"type": "email",
-				"isDefault": true,
 				"settings": {
 					"addresses": "<example@email.com>"
 				}
@@ -969,7 +967,6 @@ var brokenConfig = `
 				"uid": "abc",
 				"name": "default-email",
 				"type": "email",
-				"isDefault": true,
 				"settings": {}
 			}]
 		}]