Clarify upgrade path for OpenSearch versions

[pawelw1]

Added note about upgrading to OpenSearch 2.19 before 3.x.

Description

Current OpenSearch documentation doesn't specify a minimum 2.x version for a successful upgrade to 3.x.

Upgrade from pre-2.19 to 3.x will fail with the following error.

{“extra_data”:{“class”:“c.e.b.a.s.o.p.s.t.TlsTransport”},“message”:“Peer not connected [channel: Netty4TcpChannel{localAddress=/192.168.161.67:9300, remoteAddress=/192.168.52.103:50358}] Cause: Received message from unsupported version: [2.11.1] minimal compatible version is: [2.19.0]”,“metadata”:{“container_name”:“master”,“namespace”:“ashok”,“pod_name”:“opensearch-master-2”},“service_id”:“opensearch”,“severity”:“warning”,“timestamp”:“2025-11-20T12:15:45.180+00:00”,“version”:“1.2.0”}

Version

OpenSearch 3.x

Checklist

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[kolchfa-aws]

@sumobrian Could you review this PR?