fix(data-prepper): use official 2.16.0 image

[kangkyunghyun]

Description

Replace the default custom Data Prepper snapshot image with the official stable Data Prepper 2.16.0 image.

The previous defaults used:

• sgguruda62324/opensearch-data-prepper:2.16.0-SNAPSHOT-rc1

The new defaults use:

• opensearchproject/data-prepper:2.16.0

This change updates both the Docker Compose environment defaults and the Helm chart values so that both deployment methods use the same official image.

No Data Prepper pipeline configuration was changed.

A/B testing isolated the failure to the previously configured snapshot image. The internal cause within that image was not investigated further; this change removes the non-official snapshot default in favor of the official stable release.

Validation

• Reproduced the sink initialization failure with the previous snapshot image on a fresh OpenSearch volume.
• Repeated the clean startup test three times with the official image; all OpenSearch sinks initialized without Request execution cancelled.
• Ran the default Docker Compose stack and verified log, trace, and service-map ingestion.
• Verified the rendered Docker Compose image with docker compose config.
• Ran helm lint successfully.
• Verified the rendered Helm image with helm template.
• Ran all Helm unit tests: 73 passed.

Issues Resolved

Closes #407

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