[Cleanup] Remove upgrade-cli tool and related build references

[BeomSeogKim]

Description

The upgrade-cli tool is deprecated and no longer needed.
This PR removes the tool and all associated files and build references.

• Removed: distribution/tools/upgrade-cli
• Cleaned up: settings.gradle, distribution/build.gradle
• Deleted: distribution/src/bin/opensearch-upgrade(.bat)

Related Issues

Resolves #18390

Check List

• [x] Functionality includes testing.
• [ ] API changes companion pull request created, if applicable.
• [ ] Public documentation issue/PR created, if applicable.

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[github-actions[bot]]

:x: Gradle check result for a4db919e530f762c1587783cbc4e2f3ce17231f7: 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[bot]]

:grey_exclamation: Gradle check result for fac8be01057221bed5a0aa9d6d7e3a4ed5391433: UNSTABLE

Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 72.63%. Comparing base (7c8f01b) to head (fac8be0). Report is 77 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #18494      +/-   ##
============================================
- Coverage     72.68%   72.63%   -0.06%     
+ Complexity    68130    68032      -98     
============================================
  Files          5540     5529      -11     
  Lines        313379   313009     -370     
  Branches      45472    45424      -48     
============================================
- Hits         227785   227349     -436     
- Misses        67070    67183     +113     
+ Partials      18524    18477      -47     

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