[Backport 2.x] Replace terminology 'master' with 'cluster manager'

[opensearch-trigger-bot]

Backport 268bc67 from #521

[codecov-commenter]

Codecov Report

Merging #522 (268bc67) into 2.x (bab0790) will decrease coverage by 0.02%.
The diff coverage is 85.19%.

❗ Current head 268bc67 differs from pull request most recent head b88c5d4. Consider uploading reports for the commit b88c5d4 to get more accurate results

@@             Coverage Diff              @@
##                2.x     #522      +/-   ##
============================================
- Coverage     84.04%   84.02%   -0.03%     
+ Complexity     1019     1018       -1     
============================================
  Files           146      146              
  Lines          4188     4188              
  Branches        373      373              
============================================
- Hits           3520     3519       -1     
- Misses          492      493       +1     
  Partials        176      176              

Impacted Files	Coverage Δ
...n/java/org/opensearch/knn/common/KNNConstants.java	93.33% <ø> (ø)
.../main/java/org/opensearch/knn/index/IndexUtil.java	55.73% <ø> (ø)
...va/org/opensearch/knn/index/KNNCircuitBreaker.java	60.00% <0.00%> (ø)
.../main/java/org/opensearch/knn/index/KNNMethod.java	100.00% <ø> (ø)
...ava/org/opensearch/knn/index/KNNMethodContext.java	92.50% <ø> (ø)
...java/org/opensearch/knn/index/MethodComponent.java	88.88% <ø> (ø)
...g/opensearch/knn/index/MethodComponentContext.java	91.13% <ø> (ø)
...earch/knn/index/memory/NativeMemoryAllocation.java	82.92% <ø> (ø)
.../java/org/opensearch/knn/index/query/KNNQuery.java	83.33% <ø> (ø)
...org/opensearch/knn/index/query/KNNQueryResult.java	100.00% <ø> (ø)
... and 67 more

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