revert the routingTable.indicesRouting.keySet change

[ansjcy]

Is your feature request related to a problem? Please provide an existing Issue # , or describe.
Revert the routingTable.indicesRouting.keySet change for 2.8 release since the necessary OpenSearch side changes are not in yet.

Additional context
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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

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[codecov]

Codecov Report

Merging #469 (8103fc7) into 2.x (a465969) will increase coverage by 2.55%.
The diff coverage is 36.00%.

❗ Current head 8103fc7 differs from pull request most recent head 212026c. Consider uploading reports for the commit 212026c to get more accurate results

@@             Coverage Diff              @@
##                2.x     #469      +/-   ##
============================================
+ Coverage     70.96%   73.51%   +2.55%     
+ Complexity      378      374       -4     
============================================
  Files            44       44              
  Lines          2645     2477     -168     
  Branches        176      172       -4     
============================================
- Hits           1877     1821      -56     
+ Misses          659      547     -112     
  Partials        109      109              

Impacted Files	Coverage Δ
...performanceanalyzer/PerformanceAnalyzerPlugin.java	76.92% <ø> (ø)
...alyzer/action/PerformanceAnalyzerActionFilter.java	96.87% <ø> (ø)
...yzer/action/PerformanceAnalyzerActionListener.java	96.87% <ø> (ø)
...tors/ClusterManagerThrottlingMetricsCollector.java	0.00% <ø> (-20.00%)	⬇️
...zer/collectors/FaultDetectionMetricsCollector.java	0.00% <ø> (ø)
...nalyzer/config/setting/ClusterSettingsManager.java	5.88% <ø> (ø)
.../handler/ConfigOverridesClusterSettingHandler.java	99.02% <ø> (ø)
...config/PerformanceAnalyzerClusterConfigAction.java	85.96% <ø> (ø)
...action/config/PerformanceAnalyzerConfigAction.java	93.18% <ø> (ø)
...rformanceAnalyzerOverridesClusterConfigAction.java	80.95% <ø> (ø)
... and 20 more