Update eck-elasticsearch default secureSettings values to be slice.

[naemono]

When installing/upgrading and setting secureSettings for the eck-elasticsearch Helm chart you get this warning:

❯ helm upgrade -i my-deployment elastic/eck-stack -n elastic --values values-initial.yaml
Release "my-deployment" does not exist. Installing it now.
coalesce.go:286: warning: cannot overwrite table with non table for eck-stack.eck-elasticsearch.secureSettings (map[])
NAME: my-deployment
LAST DEPLOYED: Tue Dec 19 10:17:24 2023

With these values:

eck-elasticsearch:
  fullnameOverride: elasticsearch01
  version: 8.11.1
  secureSettings:
    - secretName: gcs-credentials
  nodeSets:
  - name: master-data-nodes
    count: 3
    config:
      node.roles: ["master", "data", "ingest"]
    volumeClaimTemplates:
    - metadata:
        name: elasticsearch-data
      spec:
        accessModes:
        - ReadWriteOnce
        resources:
          requests:
            storage: 5Gi
        storageClassName: standard

This is because secureSettings is supposed to be an array/slice, not a map.

After this update, no error is seen

❯ helm upgrade -i my-deployment . -n elastic --values /path/to/values-initial.yaml
Release "my-deployment" does not exist. Installing it now.
NAME: my-deployment
LAST DEPLOYED: Tue Dec 19 10:28:20 2023
NAMESPACE: elastic
STATUS: deployed
REVISION: 1
TEST SUITE: None
NOTES:
Elasticsearch ECK-Stack 0.9.0-SNAPSHOT has been deployed successfully!