Clear change log after 3.8 release (#1908) (#1914)

(cherry picked from commit f9c1bc0)

Signed-off-by: Heemin Kim <heemin@amazon.com>
Signed-off-by: opensearch-ci-bot <opensearch-infra@amazon.com>
Co-authored-by: Heemin Kim <heemin@amazon.com>

Author: opensearch-ci-bot

CHANGELOG.md

@@ -8,17 +8,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 ### Features
 
 ### Enhancements
-* Improve hybrid query filter validation error message ([#1870](https://github.com/opensearch-project/neural-search/pull/1870))
 
 ### Bug Fixes
-* Add `previous_score_field` to `by_field` rerank processor to avoid overwriting existing document fields ([#1880](https://github.com/opensearch-project/neural-search/pull/1880)) (OpenSearch [#21440](https://github.com/opensearch-project/OpenSearch/issues/21440))
-* [Hybrid Query] Block hybrid query execution with `search_type=dfs_query_then_fetch` ([#1873](https://github.com/opensearch-project/neural-search/pull/1873))
-* [Hybrid Query] Fix `hybrid_score_explanation` returning a single normalization block for hybrid queries on indices that contain a nested field ([#1876](https://github.com/opensearch-project/neural-search/pull/1876))
-* [Two-phase] Skip two phase rescore for sort query ([#1898](https://github.com/opensearch-project/neural-search/pull/1898))
 
 ### Infrastructure
-* Fix `Check Workflow Events` CI job: allowlist `pr_review.yml` for its required `pull_request_target` event and remove a stray fragment that broke YAML parsing of the BWC workflow ([#1901](https://github.com/opensearch-project/neural-search/pull/1901))
-* [Hybrid Query] Fix flaky `HybridQueryExplainIT` explanation test by asserting per-document explanation trees keyed by `_id` instead of hit position ([#1899](https://github.com/opensearch-project/neural-search/pull/1899))
 
 
 ### Documentation