Update approximate_threshold to 15K documents

[VijayanB]

Description

After comparing indexing and search performance, we are updating default value to be 15000.
Updated bwc rolling upgrade test and restart test to reset threshold to 0 to build graph always. Few tests checks
for graph in cache and having 15K will break that test. Hence, when new cluster comes up, we will update the setting to 0, force merge if require, before doing any new indexing.

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.

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.

[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-2229-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 eb0a3c7454cb33346f135161beb21f46f43b8457
# Push it to GitHub
git push --set-upstream origin backport/backport-2229-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-2229-to-2.x.