github webhooks: Fix message for unsupported team payloads.

The main thing fixed here is that we weren't turning
on our keys into a list.  And then I refined the message
a bit more, including sorting the keys.

I also avoid the unnecessary "else".

zerver/webhooks/github/tests.py

@@ -452,4 +452,8 @@ A temporary team so that I can get some webhook fixtures!
         stack_info = m.call_args[1]["stack_info"]
 
         self.assertIn("content_type: application/json", msg)
+        self.assertIn(
+            "summary: The 'team/edited (changes: bogus_key1/bogus_key2)' event isn't currently supported by the GitHub webhook",
+            msg,
+        )
         self.assertTrue(stack_info)