[Backport 2.x] print reason why parent task was cancelled

[opensearch-trigger-bot]

Backport dfb8449 from #14604.

[github-actions]

❌ Gradle check result for d87e3c9: 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 d87e3c9: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 71.45%. Comparing base (0142fab) to head (d87e3c9).
Report is 2 commits behind head on 2.x.

Files	Patch %	Lines
...rc/main/java/org/opensearch/tasks/TaskManager.java	40.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x   #14710      +/-   ##
============================================
- Coverage     71.62%   71.45%   -0.17%     
+ Complexity    62542    62430     -112     
============================================
  Files          5106     5106              
  Lines        293480   293483       +3     
  Branches      42761    42762       +1     
============================================
- Hits         210201   209715     -486     
- Misses        65626    66189     +563     
+ Partials      17653    17579      -74     

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