Select index settings based on cluster version

[VijayanB]

Description

With #2229 , starting 2.18 no graphs will be built if number of document is less than 15K by using default knn index settings. Hence, select index settings based on cluster version, where starting 2.18 it will have "index.knn.advancend.approximate_threshold" set to 0, and uses old defaults for any other cluster that are before 2.18.

Related Issues

#1942

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 1

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-2.x 2.x
# Navigate to the new working tree
cd .worktrees/backport-2.x
# Create a new branch
git switch --create backport/backport-2236-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 cbf90f5a456108fa49ffb85020f3edf7e307420e
# Push it to GitHub
git push --set-upstream origin backport/backport-2236-to-2.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-2236-to-2.x.