[Backport 2.x] Remove Concurrent Segment Search feature flags for feature launch (#12074)

[jed326]

Description

Manual backport for #12074

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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[jed326]

@reta @andrross here's the backport 2.x for #12074. Updated the bwc versions to 2.12 for the yaml rest-api-spec tests here too.

[github-actions]

Compatibility status:

Checks if related components are compatible with change 0a05505

Incompatible components

Incompatible components: [https://github.com/opensearch-project/performance-analyzer.git, https://github.com/opensearch-project/performance-analyzer-rca.git]

Skipped components

Compatible components

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

[github-actions]

❕ Gradle check result for 0a05505: UNSTABLE

• TEST FAILURES:

      2 org.opensearch.cluster.coordination.AwarenessAttributeDecommissionIT.testConcurrentDecommissionAction
      2 org.opensearch.cluster.MinimumClusterManagerNodesIT.testThreeNodesNoClusterManagerBlock

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

Attention: 13 lines in your changes are missing coverage. Please review.

Comparison is base (4faa3eb) 71.05% compared to head (0a05505) 71.08%.

Files	Patch %	Lines
.../opensearch/rest/action/cat/RestIndicesAction.java	50.00%	0 Missing and 8 partials ⚠️
...rg/opensearch/rest/action/cat/RestNodesAction.java	50.00%	0 Missing and 4 partials ⚠️
.../main/java/org/opensearch/search/SearchModule.java	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x   #12152      +/-   ##
============================================
+ Coverage     71.05%   71.08%   +0.03%     
- Complexity    59441    59469      +28     
============================================
  Files          4894     4894              
  Lines        280304   280284      -20     
  Branches      41115    41105      -10     
============================================
+ Hits         199161   199238      +77     
+ Misses        64284    64147     -137     
- Partials      16859    16899      +40     

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[reta]

@jed326 could you please bring these test version changes to main as well? Thank you