add release note 2.8.0 (#153) (#155)

Signed-off-by: Mingshi Liu <mingshl@amazon.com>
(cherry picked from commit 174ae5f)

Co-authored-by: Mingshi Liu <113382730+mingshl@users.noreply.github.com>

release-notes/search-processor.release-notes-2.8.0.md

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+## Version 2.8.0.0 Release Notes
+
+Compatible with OpenSearch 2.8.0
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+
+### Features
+* Add KendraRankingResponseProcessor ([#137](https://github.com/opensearch-project/search-processor/pull/137))
+* Personalized intelligent ranking for open search requests  ([#138](https://github.com/opensearch-project/search-processor/pull/138))