Bump 1.9 to 1.9.1

[IanHoang]

Description

Post 1.9.0 release. Bump Patch Version

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[opensearch-trigger-bot[bot]]

The backport to 1.9 failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/backport-1.9 1.9
# Navigate to the new working tree
pushd ../.worktrees/backport-1.9
# Create a new branch
git switch --create backport/backport-624-to-1.9
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 5c071bcd6367bb1d0916095d198c61fc9033229f
# Push it to GitHub
git push --set-upstream origin backport/backport-624-to-1.9
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/backport-1.9

Then, create a pull request where the base branch is 1.9 and the compare/head branch is backport/backport-624-to-1.9.