openapi: Use SuccessDescription in new get-stream-email-address.

This allows it to inherit the required `description` field.  This is
necessary for 7.x but not main because 6e119842bd was backported
across #26509.

zerver/openapi/zulip.yaml

@@ -15710,6 +15710,7 @@ paths:
               schema:
                 allOf:
                   - $ref: "#/components/schemas/JsonSuccessBase"
+                  - $ref: "#/components/schemas/SuccessDescription"
                   - additionalProperties: false
                     properties:
                       result: {}