Alerting: Update Discord settings to treat 'url' as a secure setting (#69588)

* make discord url secure

* support migrating unsecure settings to secure settings

* Update public/app/features/alerting/unified/utils/receiver-form.ts

Co-authored-by: William Wernert <william.wernert@grafana.com>

---------

Co-authored-by: Gilles De Mey <gilles.de.mey@gmail.com>
Co-authored-by: William Wernert <william.wernert@grafana.com>

pkg/services/ngalert/notifier/channels_config/available_channels.go

@@ -997,6 +997,7 @@ func GetAvailableNotifiers() []*NotifierPlugin {
 					Placeholder:  "Discord webhook URL",
 					PropertyName: "url",
 					Required:     true,
+					Secure:       true,
 				},
 				{
 					Label:        "Avatar URL",