Remove searchable_snapshot.extended_compatibility experimental feature

[HyunSangHan]

Description

Removed the experimental searchable_snapshot.extended_compatibility feature flag and all related functionality. (This feature was designed to allow Searchable Snapshots to read snapshots older than the guaranteed backward compatibility for OpenSearch on a best effort basis.)

The removal simplifies the codebase by eliminating experimental functionality that was not widely adopted and reduces complexity in the searchable snapshot implementation.

Related Issues

Resolves #18592

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[github-actions]

❌ Gradle check result for ef2dc57: FAILURE

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?

[msfroh]

Thanks, @HyunSangHan -- it's nice to see so much red in the diff! 😁

I've retried the failing checks, because they look like random flaky tests, not related to your change.

[github-actions]

✅ Gradle check result for ef2dc57: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 72.80%. Comparing base (b609a50) to head (2e78acd).
Report is 36 commits behind head on main.

Files with missing lines	Patch %	Lines
.../main/java/org/opensearch/index/IndexSettings.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##               main   #18735   +/-   ##
=========================================
  Coverage     72.80%   72.80%           
- Complexity    68535    68557   +22     
=========================================
  Files          5572     5573    +1     
  Lines        314779   314781    +2     
  Branches      45691    45687    -4     
=========================================
+ Hits         229166   229174    +8     
- Misses        67014    67082   +68     
+ Partials      18599    18525   -74     

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[andrross]

@msfroh I just pushed a commit to appease the breaking changes check

[github-actions]

❌ Gradle check result for 2e78acd: FAILURE

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?

[github-actions]

✅ Gradle check result for 2e78acd: SUCCESS

[msfroh]

Whoa... I thought I had merged this last week. Sorry about that. Just finally merged this now.