[release-20.0] bugfix: don't treat join predicates as filter predicates (#16472)

[vitess-bot]

Description

This is a backport of #16472

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 68.66%. Comparing base (d968111) to head (a8590aa).

Files	Patch %	Lines
go/vt/vtgate/planbuilder/operators/apply_join.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           release-20.0   #16475   +/-   ##
=============================================
  Coverage         68.65%   68.66%           
=============================================
  Files              1541     1541           
  Lines            197850   197848    -2     
=============================================
+ Hits             135835   135845   +10     
+ Misses            62015    62003   -12     

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