Alerting frontend : fix line notifier (#37744)

- Fixes #37425
- change `line` type string to uppercase

pkg/tests/api/alerting/api_notification_channel_test.go

@@ -723,7 +723,7 @@ const alertmanagerConfig = `
         "grafana_managed_receiver_configs": [
           {
             "name": "line_test",
-            "type": "line",
+            "type": "LINE",
             "settings": {},
             "secureSettings": {
               "token": "mysecrettoken"
@@ -1217,7 +1217,7 @@ var expAlertmanagerConfigFromAPI = `
           {
             "uid": "",
             "name": "line_test",
-            "type": "line",
+            "type": "LINE",
             "disableResolveMessage": false,
             "settings": {},
             "secureFields": {