Alerting: replace usage of simplejson to json.RawMessage in NotificationChannelConfig (#60423)

* introduce alias for json.RawMessage with name RawMessage. This is needed to keep raw JSON and implement a marshaler for YAML, which does not seem to be used but there are tests that fail.
* replace usage of simplejson with RawMessage in NotificationChannelConfig
* remove usage of simplejson in tests
* change migration code to convert simplejson to raw message

pkg/services/sqlstore/migrations/ualert/ualert.go

@@ -470,6 +470,10 @@ func (m *migration) validateAlertmanagerConfig(orgID int64, config *PostableUser
 				secureSettings[k] = d
 			}
 
+			data, err := gr.Settings.MarshalJSON()
+			if err != nil {
+				return err
+			}
 			var (
 				cfg = &channels.NotificationChannelConfig{
 					UID:                   gr.UID,
@@ -477,10 +481,9 @@ func (m *migration) validateAlertmanagerConfig(orgID int64, config *PostableUser
 					Name:                  gr.Name,
 					Type:                  gr.Type,
 					DisableResolveMessage: gr.DisableResolveMessage,
-					Settings:              gr.Settings,
+					Settings:              data,
 					SecureSettings:        secureSettings,
 				}
-				err error
 			)
 
 			// decryptFunc represents the legacy way of decrypting data. Before the migration, we don't need any new way,