Add securitycontext to keystore container

[cthtrifork]

Description

This fixes the issue found in this kubernetes event log:

create Pod opensearch-data-0 in StatefulSet opensearch-data failed error: pods "opensearch-data-0" is forbidden: violates PodSecurity "restricted:latest": allowPrivilegeEscalation != false (container "keystore" must set securityContext.allowPrivilegeEscalation=false), unrestricted capabilities (container "keystore" must set securityContext.capabilities.drop=["ALL"])  

When we are running

labels:
    pod-security.kubernetes.io/enforce: restricted

Issues Resolved

List any issues this PR will resolve, e.g. Closes [...].

Check List

• Commits are signed per the DCO using --signoff
• Unittest added for the new/changed functionality and all unit tests are successful
• Customer-visible features documented
• No linter warnings (make lint)

If CRDs are changed:

• CRD YAMLs updated (make manifests) and also copied into the helm chart
• Changes to CRDs documented

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[swoehrl-mw]

LGTM