[Backport 2.x] Change pinned timestamp file format stored in remote store

[opensearch-trigger-bot]

Backport cfcfe21 from #15526.

[github-actions]

❕ Gradle check result for 4332a4b: UNSTABLE

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

Attention: Patch coverage is 28.57143% with 45 lines in your changes missing coverage. Please review.

Project coverage is 71.72%. Comparing base (834b7c1) to head (4332a4b).
Report is 1 commits behind head on 2.x.

Files with missing lines	Patch %	Lines
...remotestore/RemoteStorePinnedTimestampService.java	25.00%	43 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                2.x   #15587      +/-   ##
============================================
- Coverage     71.73%   71.72%   -0.02%     
+ Complexity    63966    63949      -17     
============================================
  Files          5241     5239       -2     
  Lines        299111   299071      -40     
  Branches      43539    43539              
============================================
- Hits         214569   214495      -74     
+ Misses        66626    66606      -20     
- Partials      17916    17970      +54     

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[sachinpkale]

CodeCov is less because:

• Most of the changes in this PR contains deleting the files/code.
• Changes are covered as part of integ tests that CodeCov does not consider.