hub: add webhook callback note (#23962)

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## Description

Added note that callback is legacy and is no longer supported.

## Related issues or tickets

Closes #23955
https://docker.slack.com/archives/C01043ZHY6L/p1768249016247099

## Reviews

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- [ ] Editorial review
- [X] Technical review

Signed-off-by: Craig Osterhout <craig.osterhout@docker.com>

Author: craig-osterhout

content/manuals/docker-hub/repos/manage/webhooks.md

@@ -57,3 +57,7 @@ Webhook payloads have the following JSON format:
   }
 }
 ```
+
+> [!NOTE]
+>
+> The `callback_url` field is a legacy field and is no longer supported.