[Segment Replication] Add changelog entry and fix allocation test

[dreamer-89]

Description

This change adds the changelog entry for PR #6017 which introduces a dynamic setting exposed to end user and thus deserves entry in changelog.
This PR also uses this opportunity to fix one issue in allocation test to use replicaCount for number of replicas instead of constant.

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: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/10891/
• CommitID: 884adad

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/10892/
• CommitID: 507a9ec
  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/10892/
• CommitID: 507a9ec
  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?

Unrelated failure. Retrying gradle check

1: Task failed with an exception.
-----------
* What went wrong:
Execution failed for task ':test:fixtures:gcs-fixture:composeUp'.
> Exit-code 1 when calling /usr/bin/docker-compose, stdout: Step 1/11 : FROM ubuntu:18.04
  18.04: Pulling from library/ubuntu
  Digest: sha256:a3765b4d74747b5e9bdd03205b3fbc4fa19a02781c185f97f24c8f4f84ed7bbf
  Status: Downloaded newer image for ubuntu:18.04
   ---> 5d2df19066ac
  Step 2/11 : RUN apt-get update -qqy
   ---> Running in 598a6b7426cb
  Removing intermediate container 598a6b7426cb
   ---> db36509a16f1
  Step 3/11 : RUN apt-get install -qqy openjdk-11-jre-headless
   ---> Running in 5de154369c21
  �[91mE: Failed to fetch http://archive.ubuntu.com/ubuntu/pool/main/g/glibc/multiarch-support_2.27-3ubuntu1.6_amd64.deb  Connection failed [IP: 185.125.190.36 80]
  E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?
  �[0m

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/10895/
• CommitID: 507a9ec
  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/10895/
• CommitID: 507a9ec
  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?

Segrep test failure. Created #6259 to track this test failure. Retrying!

> Task :server:internalClusterTest

REPRODUCE WITH: ./gradlew ':server:internalClusterTest' --tests "org.opensearch.action.admin.ClientTimeoutIT.testSegmentReplicationStatsWithTimeout" -Dtests.seed=9A395F8DCBA089E7 -Dtests.security.manager=true -Dtests.jvm.argline="-XX:TieredStopAtLevel=1 -XX:ReservedCodeCacheSize=64m" -Dtests.locale=sr-Latn -Dtests.timezone=Pacific/Chuuk -Druntime.java=19

org.opensearch.action.admin.ClientTimeoutIT > testSegmentReplicationStatsWithTimeout FAILED
    java.lang.AssertionError: timed out waiting for green state
        at __randomizedtesting.SeedInfo.seed([9A395F8DCBA089E7:66682A8BCC3D53E]:0)
        at org.junit.Assert.fail(Assert.java:89)
        at org.opensearch.test.OpenSearchIntegTestCase.ensureColor(OpenSearchIntegTestCase.java:1001)
        at org.opensearch.test.OpenSearchIntegTestCase.ensureGreen(OpenSearchIntegTestCase.java:932)
        at org.opensearch.test.OpenSearchIntegTestCase.ensureGreen(OpenSearchIntegTestCase.java:921)
        at org.opensearch.action.admin.ClientTimeoutIT.testSegmentReplicationStatsWithTimeout(ClientTimeoutIT.java:177)


Suite: Test class org.opensearch.action.admin.ClientTimeoutIT
  1> [2023-02-09T12:16:46,210][INFO ][o.o.a.a.ClientTimeoutIT  ] [testNodesInfoTimeout] before test

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/10912/
• CommitID: 507a9ec