[2.x] Extend the version range to run flat-object field REST Yaml test on 2.7.0

[mingshl]

Description

Fix Auto Backport ticket #7052: Update version to 2.7.0 version in IT tests

Issues Resolved

#1018

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.
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[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❕
• TEST FAILURES:

      1 org.opensearch.indices.replication.SegmentReplicationIT.testScrollCreatedOnReplica
      1 org.opensearch.indices.replication.SegmentReplicationIT.testPitCreatedOnReplica
      1 org.opensearch.backwards.MixedClusterClientYamlTestSuiteIT.test {p0=pit/10_basic/Delete all}

• URL: https://build.ci.opensearch.org/job/gradle-check/13809/
• CommitID: 89b7af1
  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 #7081 (89b7af1) into 2.x (19e66c6) will increase coverage by 0.02%.
The diff coverage is n/a.

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@@             Coverage Diff              @@
##                2.x    #7081      +/-   ##
============================================
+ Coverage     70.31%   70.34%   +0.02%     
- Complexity    59539    59567      +28     
============================================
  Files          4824     4824              
  Lines        286330   286330              
  Branches      41626    41626              
============================================
+ Hits         201345   201405      +60     
+ Misses        68196    68033     -163     
- Partials      16789    16892     +103     

see 523 files with indirect coverage changes

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