[Backport] [2.x] Bump com.diffplug.spotless from 6.11.0 to 6.15.0 (#6379)

[reta]

Backport of #6379 to 2.x

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/11412/
• CommitID: 0ae5526
  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 (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/11413/
• CommitID: 925647e

[codecov-commenter]

Codecov Report

Merging #6385 (925647e) into 2.x (2270559) will increase coverage by 0.02%.
The diff coverage is 0.00%.

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@@             Coverage Diff              @@
##                2.x    #6385      +/-   ##
============================================
+ Coverage     70.39%   70.42%   +0.02%     
+ Complexity    59200    59186      -14     
============================================
  Files          4795     4795              
  Lines        284135   284137       +2     
  Branches      41310    41311       +1     
============================================
+ Hits         200014   200094      +80     
+ Misses        67393    67261     -132     
- Partials      16728    16782      +54     

Impacted Files	Coverage Δ
...java/org/opensearch/index/shard/StoreRecovery.java	65.34% <0.00%> (-1.02%)	⬇️
.../java/org/opensearch/plugins/IndexStorePlugin.java	0.00% <0.00%> (-100.00%)	⬇️
...main/java/org/opensearch/node/NodeMocksPlugin.java	0.00% <0.00%> (-100.00%)	⬇️
...va/org/opensearch/test/store/MockFSIndexStore.java	0.00% <0.00%> (-73.92%)	⬇️
.../java/org/opensearch/node/NodeClosedException.java	50.00% <0.00%> (-50.00%)	⬇️
.../opensearch/test/store/MockFSDirectoryFactory.java	21.33% <0.00%> (-48.00%)	⬇️
...cluster/coordination/PendingClusterStateStats.java	20.00% <0.00%> (-48.00%)	⬇️
...r/src/main/java/org/opensearch/http/HttpStats.java	21.05% <0.00%> (-47.37%)	⬇️
...nsearch/rest/action/cat/RestCatRecoveryAction.java	43.61% <0.00%> (-42.56%)	⬇️
...h/action/ingest/SimulateDocumentVerboseResult.java	60.71% <0.00%> (-39.29%)	⬇️
... and 460 more

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