DEV: Add deprecation warning for user_badge_removed event (#21255)

`user_badge_removed` event has been replaced with `user_badge_revoked`

lib/discourse_event.rb

@@ -21,6 +21,16 @@ class DiscourseEvent
         raise_error: true,
       )
     end
+
+    if event_name == :user_badge_removed
+      Discourse.deprecate(
+        "The :user_badge_removed event is deprecated. Please use :user_badge_revoked instead",
+        since: "3.1.0.beta5",
+        drop_from: "3.2.0.beta1",
+        output_in_test: true,
+      )
+    end
+
     events[event_name] << block
   end