[Backport 2.18] Downgrade version to 2.18.0 for ser/de of new parameters of RequestRequest

[opensearch-trigger-bot]

Backport 936cdb9 from #16472.

[dbwiddis]

LTGM. In 2.18.0 these are precisely the same value but if we need to bump the 2.18 branch to version 2.18.1 this would be needed for BWC.

It's OK to wait until 2.18.1 version bump on this branch to merge, though.

[github-actions]

❌ Gradle check result for 6c8e83d: 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 6c8e83d: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 71.83%. Comparing base (d27ee4e) to head (6c8e83d).
Report is 2 commits behind head on 2.18.

Files with missing lines	Patch %	Lines
...ster/snapshots/restore/RestoreSnapshotRequest.java	0.00%	0 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##               2.18   #16477   +/-   ##
=========================================
  Coverage     71.83%   71.83%           
+ Complexity    65381    65343   -38     
=========================================
  Files          5312     5312           
  Lines        304995   304995           
  Branches      44440    44440           
=========================================
+ Hits         219090   219104   +14     
+ Misses        67614    67608    -6     
+ Partials      18291    18283    -8     

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[dbwiddis]

Reminder: merge this when the below PR is merged:

• [AUTO] Increment version to 2.18.1. #16571

[reta]

2.18.0 has already sailed, does this bug require patch release?

[dbwiddis]

2.18.0 has already sailed, does this bug require patch release?

Nope, this only exists for BWC in the case a user might have to do a rolling upgrade from 2.18.0 to 2.18.1 (if it ever gets released). Which is why we're waiting for the 2.18.1 version bump to merge it.

Also, it's not a bug, it's an apparently common pattern of adding a new feature with version checks using "CURRENT" and waiting for the backport (since it wouldn't compile otherwise) and then once it's available in the backport changing to actual version numbers. There should be a better way to do that, but it is what it is.

[dbwiddis]

Given the headaches with the other Current -> 2.18 PR, going ahead and merging this now, before the version bump (thinking it could possibly break a BWC test).