Bumping the OpenSearch Client in general and the Server in the DataNode to 2.15

[janheise]

Description

Bumping our OpenSearch usage to 2.15:

• client used to connect to OpenSearch (does in general not restrict the server version used)
• server inside the DataNode

Background is: the core team works on a feature that needs the fix in opensearch-project/OpenSearch#12812 which is only available in the client from 2.15 on.

And in general, we tested 2.15 a bit more than other versions because 2.14 had some bugs so I think it's a good step to bump the versions from 2.12 to 2.15 so that it's clear that everyone should stay away from that buggy version.

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.

[todvora]

Integration tests are working, I don't see any problem with this.