Add maintenance note to Webhooks classic (#4313)

Author: kdaswani

src/connections/destinations/catalog/webhooks/index.md

@@ -1,6 +1,7 @@
 ---
 rewrite: true
 title: Webhooks Destination
+maintenance: true
 id: 54521fdc25e721e32a72ef04
 ---
 Segment Webhooks submit real-time user data to your own HTTP endpoints. A Webhook is an HTTP callback: a simple event-notification using HTTP POST. A web application implementing Webhooks will POST a message to a URL when certain things happen.