Add more yaml rest tests for search backpressure settings

[gaobinlong]

Description

Search backpressure feature has more than 10 cluster level settings but has less yaml rest test cases for them, even though we've added some tests in this PR:https://github.com/opensearch-project/OpenSearch/pull/15501/files, but it doesn't cover all settings, this PR aims to add yaml rest tests for all search backpressure settings, and move the tests from the directory cluster.put_setting to its own directory search.backpressure.

Besides, correct the supported version in the test case Test set invalid search backpressure mode from 2.9.0 to 2.8.0.

Changelog is not needed, backport to 2.x is needed.

Related Issues

No issue.

Check List

- [ ] Functionality includes testing.
- [ ] API changes companion pull request created, if applicable.
- [ ] Public documentation issue/PR created, if applicable.

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[github-actions]

✅ Gradle check result for c6d2768: SUCCESS

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.07%. Comparing base (1bc81d3) to head (c6d2768).
Report is 7 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #16012      +/-   ##
============================================
+ Coverage     71.96%   72.07%   +0.11%     
- Complexity    64391    64460      +69     
============================================
  Files          5280     5280              
  Lines        300866   300871       +5     
  Branches      43474    43474              
============================================
+ Hits         216510   216848     +338     
+ Misses        66639    66275     -364     
- Partials      17717    17748      +31     

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[opensearch-trigger-bot]

The backport to 2.x failed:

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

To backport manually, run these commands in your terminal:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/OpenSearch/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/OpenSearch/backport-2.x
# Create a new branch
git switch --create backport/backport-16012-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 4963792c175ab3eb30a493e81360dce95b5b2811
# Push it to GitHub
git push --set-upstream origin backport/backport-16012-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/OpenSearch/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-16012-to-2.x.