Add release notes for v2.6.0

[kartg]

Description

This change adds release notes for the v2.6.0 release and cleans up the CHANGELOG to only capture unreleased changes. The change also includes minor language updates.

Issues Resolved

relates #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: UNSTABLE ❕
• TEST FAILURES:

      1 org.opensearch.cluster.routing.allocation.decider.DiskThresholdDeciderIT.testIndexCreateBlockIsRemovedWhenAnyNodesNotExceedHighWatermarkWithAutoReleaseEnabled

• URL: https://build.ci.opensearch.org/job/gradle-check/11589/
• CommitID: fe32a77
  Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/11590/
• CommitID: 406e307
  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?

[kartg]

REPRODUCE WITH: ./gradlew ':server:internalClusterTest' --tests "org.opensearch.cluster.shards.ClusterShardLimitIT.testCreateIndexWithMaxClusterShardSetting" -Dtests.seed=AE874172A4ACFE71 -Dtests.security.manager=true -Dtests.jvm.argline="-XX:TieredStopAtLevel=1 -XX:ReservedCodeCacheSize=64m" -Dtests.locale=it-CH -Dtests.timezone=Asia/Famagusta -Druntime.java=19

org.opensearch.cluster.shards.ClusterShardLimitIT > testCreateIndexWithMaxClusterShardSetting FAILED
    java.lang.AssertionError: MaxShardPerCluster 2 should be greater than or equal to MaxShardPerNode 1000
        at __randomizedtesting.SeedInfo.seed([AE874172A4ACFE71:E461B0CC09F238C9]:0)
        at org.junit.Assert.fail(Assert.java:89)
        at org.opensearch.cluster.shards.ClusterShardLimitIT.setMaxShardLimit(ClusterShardLimitIT.java:750)
        at org.opensearch.cluster.shards.ClusterShardLimitIT.testCreateIndexWithMaxClusterShardSetting(ClusterShardLimitIT.java:259)

Flaky test failure - #6287

Will refire

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❕
• TEST FAILURES:

      1 org.opensearch.search.SearchWeightedRoutingIT.testSearchAggregationWithNetworkDisruption_FailOpenEnabled

• URL: https://build.ci.opensearch.org/job/gradle-check/11594/
• CommitID: 406e307
  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 #6443 (406e307) into main (8865d58) will decrease coverage by 0.09%.
The diff coverage is n/a.

📣 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              @@
##               main    #6443      +/-   ##
============================================
- Coverage     70.69%   70.60%   -0.09%     
+ Complexity    59016    58963      -53     
============================================
  Files          4800     4800              
  Lines        282712   282712              
  Branches      40763    40763              
============================================
- Hits         199855   199602     -253     
- Misses        66473    66687     +214     
- Partials      16384    16423      +39     

Impacted Files	Coverage Δ
.../java/org/opensearch/node/NodeClosedException.java	50.00% <0.00%> (-50.00%)	⬇️
...ava/org/opensearch/action/NoSuchNodeException.java	0.00% <0.00%> (-50.00%)	⬇️
...a/org/opensearch/tasks/TaskCancelledException.java	50.00% <0.00%> (-50.00%)	⬇️
.../index/shard/IndexShardNotRecoveringException.java	50.00% <0.00%> (-50.00%)	⬇️
.../java/org/opensearch/search/dfs/AggregatedDfs.java	54.83% <0.00%> (-41.94%)	⬇️
...cluster/coordination/PublishClusterStateStats.java	33.33% <0.00%> (-37.51%)	⬇️
...arch/search/aggregations/pipeline/SimpleModel.java	38.46% <0.00%> (-30.77%)	⬇️
...tions/InternalSingleBucketAggregationTestCase.java	59.74% <0.00%> (-28.58%)	⬇️
...opensearch/index/reindex/BulkByScrollResponse.java	48.38% <0.00%> (-28.23%)	⬇️
...rc/main/java/org/opensearch/ingest/IngestInfo.java	51.72% <0.00%> (-27.59%)	⬇️
... and 480 more

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[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/backport-2.x
# Create a new branch
git switch --create backport/backport-6443-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 01466a6a04e280267f013aeb14dff1f20dc7a77a
# Push it to GitHub
git push --set-upstream origin backport/backport-6443-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-6443-to-2.x.