Fix wrong reference to existingKey (#415)

### Description of the change

Fix a wrong reference to `signing.existingKey`, `signing.existingSecret` was what was meant and what is used in the chart.

### Benefits

Less confusion when trying to use the Helm chart.

### Possible drawbacks

Evangelists of `existingKey` storming the barricades even though `existingKey` is long dead.

### Applicable issues

None, nobody noticed enough to care, apparently.

Co-authored-by: Tobias Wolter <towo@towo.eu>
Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/415
Reviewed-by: pat-s <pat-s@noreply.gitea.io>
Reviewed-by: justusbunsi <justusbunsi@noreply.gitea.io>
Co-authored-by: towo <towo@noreply.gitea.io>
Co-committed-by: towo <towo@noreply.gitea.io>

unittests/gpg-secret/signing-enabled.yaml

@@ -11,7 +11,7 @@ tests:
         enabled: true
     asserts:
       - failedTemplate:
-          errorMessage: Either specify `signing.privateKey` or `signing.existingKey`
+          errorMessage: Either specify `signing.privateKey` or `signing.existingSecret`
   - it: skips rendering using external secret reference
     set:
       signing: