[Backport 2.x] Simplify diff calculation for remote routing table

[opensearch-trigger-bot]

Backport 7247266 from #15100.

[github-actions]

❌ Gradle check result for 51e135c: FAILURE

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 result for 51e135c: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 79.82456% with 23 lines in your changes missing coverage. Please review.

Project coverage is 71.78%. Comparing base (8a9c167) to head (51e135c).
Report is 6 commits behind head on 2.x.

Files with missing lines	Patch %	Lines
...arch/gateway/remote/RemoteClusterStateService.java	60.60%	6 Missing and 7 partials ⚠️
...earch/gateway/remote/ClusterStateDiffManifest.java	86.95%	1 Missing and 2 partials ⚠️
...h/cluster/routing/RoutingTableIncrementalDiff.java	94.11%	2 Missing ⚠️
...ay/remote/model/RemoteClusterMetadataManifest.java	33.33%	1 Missing and 1 partial ⚠️
...a/org/opensearch/cluster/routing/RoutingTable.java	50.00%	1 Missing ⚠️
...search/gateway/remote/ClusterMetadataManifest.java	90.00%	1 Missing ⚠️
...ay/remote/routingtable/RemoteRoutingTableDiff.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x   #15582      +/-   ##
============================================
+ Coverage     71.64%   71.78%   +0.14%     
- Complexity    63909    64005      +96     
============================================
  Files          5240     5239       -1     
  Lines        299047   299043       -4     
  Branches      43535    43543       +8     
============================================
+ Hits         214244   214671     +427     
+ Misses        66881    66409     -472     
- Partials      17922    17963      +41     

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