Add 2.12 release notes to 2.x branch

[andrross]

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Failing checks are inspected and point to the corresponding known issue(s) (See: Troubleshooting Failing Builds)
• Commits are signed per the DCO using --signoff
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)
• Public documentation issue/PR created

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[github-actions]

Compatibility status:

Checks if related components are compatible with change 384469c

Incompatible components

Skipped components

Compatible components

Compatible components: [https://github.com/opensearch-project/custom-codecs.git, https://github.com/opensearch-project/asynchronous-search.git, https://github.com/opensearch-project/neural-search.git, https://github.com/opensearch-project/flow-framework.git, https://github.com/opensearch-project/job-scheduler.git, https://github.com/opensearch-project/cross-cluster-replication.git, https://github.com/opensearch-project/security-analytics.git, https://github.com/opensearch-project/observability.git, https://github.com/opensearch-project/reporting.git, https://github.com/opensearch-project/geospatial.git, https://github.com/opensearch-project/k-nn.git, https://github.com/opensearch-project/notifications.git, https://github.com/opensearch-project/common-utils.git, https://github.com/opensearch-project/anomaly-detection.git, https://github.com/opensearch-project/performance-analyzer-rca.git, https://github.com/opensearch-project/ml-commons.git, https://github.com/opensearch-project/index-management.git, https://github.com/opensearch-project/sql.git, https://github.com/opensearch-project/security.git, https://github.com/opensearch-project/alerting.git, https://github.com/opensearch-project/performance-analyzer.git]

[github-actions]

❕ Gradle check result for 42a7dc6: UNSTABLE

• TEST FAILURES:

      2 org.opensearch.remotestore.RemoteIndexPrimaryRelocationIT.testPrimaryRelocationWhileIndexing
      1 org.opensearch.search.SearchWeightedRoutingIT.testMultiGetWithNetworkDisruption_FailOpenEnabled

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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (74bd8f8) 71.21% compared to head (384469c) 71.16%.

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x   #12310      +/-   ##
============================================
- Coverage     71.21%   71.16%   -0.06%     
+ Complexity    60004    59939      -65     
============================================
  Files          4944     4944              
  Lines        282197   282197              
  Branches      41335    41335              
============================================
- Hits         200969   200817     -152     
- Misses        64241    64371     +130     
- Partials      16987    17009      +22     

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[github-actions]

❕ Gradle check result for 384469c: UNSTABLE

• TEST FAILURES:

      1 org.opensearch.remotestore.multipart.RemoteStoreMultipartIT.testDefaultBufferInterval
      1 org.opensearch.remotestore.RemoteIndexPrimaryRelocationIT.testPrimaryRelocationWhileIndexing

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