[Backport 2.x] Fixing SortField comparison to use equals instead of reference equality

[opensearch-trigger-bot]

Backport 55936ac from #6901.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/13324/
• CommitID: 6bf9ea8
  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: UNSTABLE ❕
• TEST FAILURES:

      1 org.opensearch.backwards.MixedClusterClientYamlTestSuiteIT.test {p0=indices.split/30_copy_settings/Copy settings during split index}

• URL: https://build.ci.opensearch.org/job/gradle-check/13326/
• CommitID: 6bf9ea8
  Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[codecov-commenter]

Codecov Report

Merging #6957 (6bf9ea8) into 2.x (5cf4c5d) will decrease coverage by 0.13%.
The diff coverage is 100.00%.

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@@             Coverage Diff              @@
##                2.x    #6957      +/-   ##
============================================
- Coverage     70.42%   70.29%   -0.13%     
+ Complexity    59500    59435      -65     
============================================
  Files          4811     4811              
  Lines        285539   285539              
  Branches      41509    41509              
============================================
- Hits         201086   200731     -355     
- Misses        67682    68037     +355     
  Partials      16771    16771              

Impacted Files	Coverage Δ
...pensearch/action/search/SearchPhaseController.java	85.42% <100.00%> (+0.28%)	⬆️

... and 463 files with indirect coverage changes

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