ci(mend-slack): run on new Discussions (#39776)

Now we've got the new Discussion form
(4c889f8) we need to make sure that the
Action triggers on creation events.

Author: jamietanna

.github/workflows/mend-slack.yml

@@ -2,7 +2,7 @@ name: 'Mend Slack Notifier'
 
 on:
   discussion:
-    types: [labeled]
+    types: [created, labeled]
 
 permissions:
   discussions: read