FIX: correctly deletes webhook_events with webhook (#23097)

Each time a message is created through a webhook, we create we webhook_event associated to this webhook.

When destroying a webhook, we were not destroying the webhook_events which was causing orphans records and more importantly errors in the app expecting to find and associated webhook.

plugins/chat/spec/models/chat/incoming_webhook_spec.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+RSpec.describe Chat::IncomingWebhook do
+  it do
+    is_expected.to have_many(:chat_webhook_events)
+      .with_foreign_key("incoming_chat_webhook_id")
+      .class_name("Chat::WebhookEvent")
+      .dependent(:delete_all)
+  end
+end