Fix supported OpenSearch versions and GitLab releases mapping

Changzheng Liurequested to merge
378958-cz-udpate-opensearch-supported-versions into master

What does this MR do and why?

We actually only started to support OpenSearch 2.x in 15.5.3 as being pointed out in this issue. So, I am updating our doc to reflect that.

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Related to #378958 (closed)

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