Update dependabot changelog helper for 3.x major version (opensearch-project#17645)

Signed-off-by: Andrew Ross <[email protected]>

Author: andrross

.github/workflows/dependabot_pr.yml

@@ -58,7 +58,7 @@ jobs:
       - name: Update the changelog
         uses: dangoslen/dependabot-changelog-helper@v4
         with:
-          version: 'Unreleased 2.x'
+          version: 'Unreleased 3.x'
 
       - name: Commit the changes
         uses: stefanzweifel/git-auto-commit-action@v5