Update LTR stats endpoint to /_plugins/_ltr/stats

[christophlee]

Description

Updates LTR stats endpoint from /_ltr/_stats to /_plugins/_ltr/stats as /_ltr/_stats was deprecated and is no longer exposed. Also updates descriptions and examples relating to this endpoint.

Related Issues/PRs

• Deprecate /_ltr/_stats endpoint and related legacy code opensearch-learning-to-rank-base#99
• Deprecating Redundant and duplicated API and package. opensearch-learning-to-rank-base#117

Version

2.19+

Checklist

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