[improve] Upgrade opensearch sink to client 2.16 and tests to use server 2.16.0 in 3.1

[nikhil-ctds]

Motivation

Upgrade opensearch sink to client 2.16 and tests to use server 2.16.0 in 3.1 branch

Modifications

Upgraded opensearch sink to client 2.16 and updated tests to use server 2.16.0 in 3.1 branch

Verifying this change

• Make sure that the change passes the CI checks.

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

NO

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API: (yes / no)
• The schema: (yes / no / don't know)
• The default values of configurations: (yes / no)
• The wire protocol: (yes / no)
• The rest endpoints: (yes / no)
• The admin cli options: (yes / no)
• Anything that affects deployment: (yes / no / don't know)

Documentation

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Need to update docs?

• doc-required

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• no-need-doc

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• doc

  (If this PR contains doc changes)