[release-20.0-rc] feat: add a LIMIT 1 on EXISTS subqueries to limit network overhead (#16153)

[vitess-bot]

Description

This is a backport of #16153

[vitess-bot]

Review Checklist

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General

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[codecov]

Codecov Report

Attention: Patch coverage is 70.58824% with 25 lines in your changes missing coverage. Please review.

Project coverage is 68.49%. Comparing base (4865893) to head (b25dcda).
Report is 2 commits behind head on release-20.0-rc.

Files	Patch %	Lines
.../vt/vtgate/planbuilder/operators/query_planning.go	67.53%	25 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##           release-20.0-rc   #16192      +/-   ##
===================================================
+ Coverage            68.47%   68.49%   +0.01%     
===================================================
  Files                 1541     1541              
  Lines               197614   197709      +95     
===================================================
+ Hits                135314   135411      +97     
+ Misses               62300    62298       -2     

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