Refactor out the old cluster state metadata logic

[bowenlan-amzn]

Issue #, if available:

Description of changes:

A follow up on #771 , clean remaining cluster state metadata logic
In TransportExplainAction, only doing a minimal change, since it's still no time for 3.0 release, need to reduce the backport friction until then.

CheckList:

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[codecov]

Codecov Report

Merging #963 (40174cc) into main (a2dd769) will increase coverage by 0.41%.
The diff coverage is 85.71%.

@@             Coverage Diff              @@
##               main     #963      +/-   ##
============================================
+ Coverage     75.55%   75.97%   +0.41%     
+ Complexity     2842     2823      -19     
============================================
  Files           364      361       -3     
  Lines         16263    16074     -189     
  Branches       2343     2314      -29     
============================================
- Hits          12288    12212      -76     
+ Misses         2626     2537      -89     
+ Partials       1349     1325      -24     

Files	Coverage Δ
...pensearch/indexmanagement/IndexManagementPlugin.kt	89.45% <ø> (-0.25%)	⬇️
...nt/indexstatemanagement/ManagedIndexCoordinator.kt	71.83% <ø> (+2.73%)	⬆️
...agement/indexstatemanagement/ManagedIndexRunner.kt	47.88% <ø> (+0.52%)	⬆️
...temanagement/opensearchapi/OpenSearchExtensions.kt	69.33% <ø> (+0.97%)	⬆️
...t/settings/LegacyOpenDistroManagedIndexSettings.kt	98.36% <ø> (-0.21%)	⬇️
...exstatemanagement/settings/ManagedIndexSettings.kt	98.36% <ø> (-0.20%)	⬇️
...action/changepolicy/TransportChangePolicyAction.kt	67.53% <100.00%> (+1.88%)	⬆️
...action/removepolicy/TransportRemovePolicyAction.kt	64.89% <ø> (-0.19%)	⬇️
...gedindex/TransportRetryFailedManagedIndexAction.kt	67.78% <100.00%> (+1.11%)	⬆️
...ent/indexstatemanagement/util/ManagedIndexUtils.kt	76.88% <ø> (+7.14%)	⬆️
... and 3 more

... and 9 files with indirect coverage changes