[Fleet] fix kafka ssl key secret update (elastic#174261)

## Summary

Closes elastic#174227

Fix wrong field used for kafka ssl key secret input which broke ssl key
on kafka output update.

To verify:
- create a kafka output with Authentication: SSL, fill out cert and key
- verify that the value of the key is stored as secret in
`.fleet-secrets`
- update kafka output (e.g. change name)
- verify that the ssl key is unchanged

<img width="1572" alt="image"
src="https://github.com/elastic/kibana/assets/90178898/3129ca89-af63-42c4-bc8d-4ff7d280e513">

x-pack/plugins/fleet/public/applications/fleet/sections/settings/components/edit_output_flyout/use_output_form.tsx

@@ -399,7 +399,7 @@ export function useOutputForm(onSucess: () => void, output?: Output) {
   );
 
   const kafkaSslKeySecretInput = useSecretInput(
-    kafkaOutput?.ssl?.certificate,
+    kafkaOutput?.secrets?.ssl?.key,
     kafkaAuthMethodInput.value === kafkaAuthType.Ssl ? validateSSLKeySecret : undefined,
     isSSLEditable
   );