[Backport 2.6] Add release notes for v2.6.0 (#6443) (#6447)

[kartg]

Signed-off-by: Kartik Ganesh gkart@amazon.com
(cherry picked from commit 01466a6) (cherry picked from commit 03a3129)

Description

• Add release notes for v2.6.0

This change also includes cleanup of the CHANGELOG file to only capture unreleased changes.

Signed-off-by: Kartik Ganesh gkart@amazon.com

• Minor language updates in v2.6.0 release notes and Changelog

Signed-off-by: Kartik Ganesh gkart@amazon.com

Issues Resolved

relates #6447 and #6360

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/11606/
• CommitID: 57fdb6f
  Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green.
  Is the failure a flaky test unrelated to your change?

[dreamer-89]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/11606/
• CommitID: 57fdb6f
  Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green.
  Is the failure a flaky test unrelated to your change?

Non-related but concerning! I see gradle check failure due to bunch of RepositoryVerificationException[[n] path is not accessible on cluster-manager node] exception. @kartg : Is this known issue ?
[Edit]: I see gradle check in progress. Let's wait for it to complete.

REPRODUCE WITH: ./gradlew ':plugins:repository-azure:internalClusterTest' --tests "org.opensearch.repositories.azure.AzureBlobStoreRepositoryTests.testList" -Dtests.seed=9062008B7EBD2B2E -Dtests.security.manager=true -Dtests.jvm.argline="-XX:TieredStopAtLevel=1 -XX:ReservedCodeCacheSize=64m" -Dtests.locale=sr-Latn-BA -Dtests.timezone=America/Resolute -Druntime.java=17

org.opensearch.repositories.azure.AzureBlobStoreRepositoryTests > testList FAILED
    RepositoryVerificationException[[n] path  is not accessible on cluster-manager node]; nested: NotSerializableExceptionWrapper[timeout_exception: Did not observe any item or terminal signal within 1000ms in 'map' (and no fallback has been configured)];
        at __randomizedtesting.SeedInfo.seed([9062008B7EBD2B2E:24C65BEBB1A16AA6]:0)
        at org.opensearch.repositories.blobstore.BlobStoreRepository.startVerification(BlobStoreRepository.java:1614)
        at org.opensearch.repositories.RepositoriesService$3.doRun(RepositoriesService.java:324)
        at org.opensearch.common.util.concurrent.ThreadContext$ContextPreservingAbstractRunnable.doRun(ThreadContext.java:806)
        at org.opensearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:52)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
        at java.lang.Thread.run(Thread.java:833)

        Caused by:
        NotSerializableExceptionWrapper[timeout_exception: Did not observe any item or terminal signal within 1000ms in 'map' (and no fallback has been configured)]
            at reactor.core.publisher.FluxTimeout$TimeoutMainSubscriber.handleTimeout(FluxTimeout.java:295)
            at reactor.core.publisher.FluxTimeout$TimeoutMainSubscriber.doTimeout(FluxTimeout.java:280)
            at reactor.core.publisher.FluxTimeout$TimeoutTimeoutSubscriber.onNext(FluxTimeout.java:419)
            at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:79)
            at reactor.core.publisher.MonoDelay$MonoDelayRunnable.propagateDelay(MonoDelay.java:271)
            at reactor.core.publisher.MonoDelay$MonoDelayRunnable.run(MonoDelay.java:286)
            at reactor.core.scheduler.SchedulerTask.call(SchedulerTask.java:68)
            at reactor.core.scheduler.SchedulerTask.call(SchedulerTask.java:28)
            at java.util.concurrent.FutureTask.run(FutureTask.java:264)
            at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:304)
            ... 3 more

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/11613/
• CommitID: 57fdb6f

[codecov-commenter]

Codecov Report

Merging #6449 (57fdb6f) into 2.6 (9977a1a) will decrease coverage by 0.09%.
The diff coverage is 30.59%.

❗ Current head 57fdb6f differs from pull request most recent head 959a302. Consider uploading reports for the commit 959a302 to get more accurate results

📣 This organization is not using Codecov’s GitHub App Integration. We recommend you install it so Codecov can continue to function properly for your repositories. Learn more

@@             Coverage Diff              @@
##                2.6    #6449      +/-   ##
============================================
- Coverage     70.55%   70.47%   -0.09%     
+ Complexity    59349    59215     -134     
============================================
  Files          4798     4796       -2     
  Lines        284205   284122      -83     
  Branches      41318    41299      -19     
============================================
- Hits         200513   200224     -289     
- Misses        66994    67116     +122     
- Partials      16698    16782      +84     

Impacted Files	Coverage Δ
...opensearch/client/slm/SnapshotLifecyclePolicy.java	0.00% <0.00%> (ø)
...ch/client/slm/SnapshotLifecyclePolicyMetadata.java	0.00% <0.00%> (ø)
.../opensearch/client/slm/SnapshotLifecycleStats.java	0.00% <0.00%> (ø)
...rch/client/slm/SnapshotRetentionConfiguration.java	0.00% <0.00%> (ø)
...java/org/opensearch/common/xcontent/MediaType.java	100.00% <ø> (ø)
...a/org/opensearch/common/xcontent/XContentType.java	100.00% <ø> (ø)
...h/script/mustache/MultiSearchTemplateResponse.java	56.25% <0.00%> (ø)
...rg/opensearch/index/rankeval/RankEvalResponse.java	98.57% <0.00%> (ø)
...va/org/opensearch/index/rankeval/RankEvalSpec.java	97.36% <0.00%> (+0.87%)	⬆️
...a/org/opensearch/index/rankeval/RatedDocument.java	81.03% <0.00%> (ø)
... and 571 more

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[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/11615/
• CommitID: 959a302