Elasticsearch 6.8 as a target

[sumobrian]

Is your feature request related to a problem?

When migrating data using the OpenSearch-Migrations tools, there is currently no support for Elasticsearch 6 as a target cluster. Supporting Elasticsearch 6 as a target would align with real-world use cases where mixed environments of OpenSearch and Elasticsearch exist, especially for legacy systems.

What solution would you like?

Enable support for Elasticsearch 6 as a target cluster within the OpenSearch-Migrations repository. This includes:

1. Updates to the reindexing process to accommodate Elasticsearch 6’s specific indexing requirements and API compatibility.
2. Ensuring metadata migrations and transformations can handle Elasticsearch 6’s structure and constraints.

What alternatives have you considered?

1. Manual Reindexing: Export data and manually reindex into Elasticsearch 6. This is error-prone and does not leverage the automated capabilities of the OpenSearch-Migrations tools. In addition it is very slow and impractical with large clusters.
2. Intermediate Migration Steps: Migrate first to an intermediate version and then to Elasticsearch 6. This is complex and increases the risk of data inconsistencies.
3. Custom Transformation Plugins: Build custom plugins for Elasticsearch 6 transformations, but this requires significant development effort and expertise.

Do you have any additional context?

• Elasticsearch 6 is still used in many production environments due to its stability and long-standing presence in the Elasticsearch ecosystem.
• Adding this support will improve adoption of the OpenSearch-Migrations tools by users in mixed-version environments.
• Supporting Elasticsearch 6 as a target complements the existing focus on supporting migrations to OpenSearch and helps bridge the gap for users with mixed infrastructure needs.
• Some users need an intermediate step in their migrations moving to ES 6.8 before moving to a later version of Elasticsearch or OpenSearch.

[peternied]

@sumobrian Could we rename this from Elasticsearch 6.8 as a target -> Support Elasticsearch 6.8 to Elasticsearch 6.8 since we aren't supported downgrades or upgrades to ES 6.8?