keep Unreleased header when updating version

Author: ezhang6811

.github/workflows/pre-release-prepare.yml

@@ -96,7 +96,7 @@ jobs:
 
       - name: Update CHANGELOG for release
         run: |
-          sed -i "s/## Unreleased/## ${VERSION} - $(date +%Y-%m-%d)/" CHANGELOG.md
+          sed -i "s/## Unreleased/## Unreleased\n\n## v${VERSION} - $(date +%Y-%m-%d)/" CHANGELOG.md
           git add CHANGELOG.md
           git commit -m "Update CHANGELOG for version ${VERSION}"
           git push origin "v${VERSION}_release"