Update to Gradle 9

[andrross]

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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[andrross]

This change depends on a gradle plugin update here: opensearch-project/OpenSearch#19400

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.53%. Comparing base (5699707) to head (32cb57f).
⚠️ Report is 1 commits behind head on main.

❌ Your project status has failed because the head coverage (70.53%) is below the target coverage (75.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #834      +/-   ##
==========================================
- Coverage   70.63%   70.53%   -0.10%     
==========================================
  Files          42       42              
  Lines        2057     2057              
  Branches      165      165              
==========================================
- Hits         1453     1451       -2     
- Misses        493      494       +1     
- Partials      111      112       +1     

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