[Backport 2.x] Update to Gradle 8.1

[opensearch-trigger-bot]

Backport 1d39b56 from #6792.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❕
• TEST FAILURES:

      1 org.opensearch.indices.replication.SegmentReplicationRelocationIT.testRelocateWhileContinuouslyIndexingAndWaitingForRefresh
      1 org.opensearch.index.translog.RemoteFSTranslogTests.testConcurrentWriteViewsAndSnapshot
      1 org.opensearch.backwards.MixedClusterClientYamlTestSuiteIT.test {p0=pit/10_basic/Delete all}

• URL: https://build.ci.opensearch.org/job/gradle-check/14420/
• CommitID: deeb892
  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-commenter]

Codecov Report

Merging #7271 (deeb892) into 2.x (df8dca7) will increase coverage by 0.03%.
The diff coverage is 0.00%.

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@@             Coverage Diff              @@
##                2.x    #7271      +/-   ##
============================================
+ Coverage     70.39%   70.43%   +0.03%     
- Complexity    59757    59784      +27     
============================================
  Files          4847     4850       +3     
  Lines        287139   287179      +40     
  Branches      41723    41727       +4     
============================================
+ Hits         202139   202269     +130     
+ Misses        68020    67951      -69     
+ Partials      16980    16959      -21     

Impacted Files	Coverage Δ
...rg/opensearch/gradle/OpenSearchTestBasePlugin.java	6.59% <0.00%> (-0.47%)	⬇️
.../org/opensearch/gradle/jvm/JvmTestSuiteHelper.java	0.00% <0.00%> (ø)
...arch/gradle/precommit/TestingConventionsTasks.java	0.00% <0.00%> (ø)
.../org/opensearch/gradle/test/RestIntegTestTask.java	0.00% <0.00%> (ø)
...earch/gradle/test/TestSuiteConventionMappings.java	0.00% <0.00%> (ø)
...main/java/org/opensearch/gradle/test/TestTask.java	0.00% <0.00%> (ø)

... and 500 files with indirect coverage changes

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