[Backport 2.x] [Search Pipeline] Flip SEARCH_PIPELINE feature flag for search pipeline GA (#8613)

[noCharger]

(cherry picked from commit 0515501)
Signed-off-by: Louis Chu clingzhi@amazon.com

Description

Manually backport #8613 to resolve merge conflict on changelog

• Commits are signed per the DCO using --signoff
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)

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[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❕
• TEST FAILURES:

      1 org.opensearch.remotestore.SegmentReplicationUsingRemoteStoreIT.testDropPrimaryDuringReplication
      1 org.opensearch.indices.replication.SegmentReplicationAllocationIT.testSingleIndexShardAllocation

• URL: https://build.ci.opensearch.org/job/gradle-check/19974/
• CommitID: b93453f
  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

Merging #8643 (b93453f) into 2.x (14e2091) will decrease coverage by 0.13%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                2.x    #8643      +/-   ##
============================================
- Coverage     70.68%   70.55%   -0.13%     
+ Complexity    57152    57112      -40     
============================================
  Files          4745     4745              
  Lines        270757   270757              
  Branches      39961    39961              
============================================
- Hits         191378   191046     -332     
- Misses        62949    63343     +394     
+ Partials      16430    16368      -62     

Impacted Files	Coverage Δ
.../java/org/opensearch/common/util/FeatureFlags.java	92.30% <100.00%> (ø)

... and 470 files with indirect coverage changes