Update knn index default to always do approximate search in test

[VijayanB]

Description

Update knn index default to always do approximate search in test cases

Related Issues

Resolves #2329

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.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[heemin32]

There might be test cases which use custom setting instead of default setting. We need to update those cases as well.

[VijayanB]

There might be test cases which use custom setting instead of default setting. We need to update those cases as well.

like what? Can you share the test that you are referring to?

[navneet1v]

for cases like this I think we should just move the code to use getDefaultIndexSettings so that in future we don't need to make the changes.

[navneet1v]

Overall looks good to me. Just 1 minor comment

[VijayanB]

There might be test cases which use custom setting instead of default setting. We need to update those cases as well.
@heemin32 I realized what you mean by custom mapping. I searched for index.knn is set true and updated those index setting with approximate threshold parameter

[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-2332-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 dc369e6d73beb068c4203db9f9375a6194d824f8
# Push it to GitHub
git push --set-upstream origin backport/backport-2332-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-2332-to-2.x.