Remove workaround to access Lucene's "expert" readLatestCommit API

[kartg]

Description

Lucene 9.6.0 includes changes to make the "expert" readLatestCommit API public, so the current code workaround using DirectoryReader is no longer required.

Related Issues

Resolves #7084

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)

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.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/15963/
• CommitID: 8dab32e
  Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green.
  Is the failure a flaky test unrelated to your change?

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/15966/
• CommitID: 8dab32e
  Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green.
  Is the failure a flaky test unrelated to your change?

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/15973/
• CommitID: 8dab32e

[codecov]

Codecov Report

Merging #7671 (8dab32e) into main (eb51f2b) will increase coverage by 0.03%.
The diff coverage is 75.00%.

@@             Coverage Diff              @@
##               main    #7671      +/-   ##
============================================
+ Coverage     70.72%   70.75%   +0.03%     
- Complexity    56044    56051       +7     
============================================
  Files          4676     4676              
  Lines        265969   265965       -4     
  Branches      39061    39061              
============================================
+ Hits         188102   188191      +89     
+ Misses        61902    61794     -108     
- Partials      15965    15980      +15     

Impacted Files	Coverage Δ
...in/java/org/opensearch/index/shard/IndexShard.java	70.09% <0.00%> (+0.32%)	⬆️
...main/java/org/opensearch/common/lucene/Lucene.java	73.93% <100.00%> (+1.22%)	⬆️
...va/org/opensearch/index/engine/ReadOnlyEngine.java	72.14% <100.00%> (ø)
...rc/main/java/org/opensearch/index/store/Store.java	79.51% <100.00%> (+0.42%)	⬆️

... and 468 files with indirect coverage changes

[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:

# 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
pushd ../.worktrees/backport-2.x
# Create a new branch
git switch --create backport/backport-7671-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 47d4f38eef90687943214e7c8847a3dacd968b1a
# Push it to GitHub
git push --set-upstream origin backport/backport-7671-to-2.x
# Go back to the original working tree
popd
# 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-7671-to-2.x.