[Backport 2.x] [Refactor] SegmentsStats#filesSizes from ImmutableOpenMap to java.util.Map

[opensearch-trigger-bot]

Backport 352de4e from #7094.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❕
• TEST FAILURES:

      1 org.opensearch.search.SearchWeightedRoutingIT.testStrictWeightedRoutingWithCustomString
      1 org.opensearch.indices.replication.SegmentReplicationIT.testScrollWithOngoingSegmentReplication
      1 org.opensearch.indices.replication.SegmentReplicationIT.testReplicationPostDeleteAndForceMerge
      1 org.opensearch.backwards.MixedClusterClientYamlTestSuiteIT.test {p0=pit/10_basic/Delete all}

• URL: https://build.ci.opensearch.org/job/gradle-check/14044/
• CommitID: 7e93a4d
  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 #7162 (7e93a4d) into 2.x (69df1c2) will increase coverage by 0.06%.
The diff coverage is 75.00%.

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@@             Coverage Diff              @@
##                2.x    #7162      +/-   ##
============================================
+ Coverage     70.33%   70.39%   +0.06%     
- Complexity    59672    59733      +61     
============================================
  Files          4847     4847              
  Lines        287100   287085      -15     
  Branches      41709    41706       -3     
============================================
+ Hits         201922   202093     +171     
+ Misses        68245    68101     -144     
+ Partials      16933    16891      -42     

Impacted Files	Coverage Δ
.../main/java/org/opensearch/index/engine/Engine.java	74.76% <40.00%> (-0.32%)	⬇️
...ava/org/opensearch/index/engine/SegmentsStats.java	72.41% <80.00%> (+1.42%)	⬆️

... and 459 files with indirect coverage changes

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