[release-23.0] Fix column offset tracking for UNIONs to be case insensitive. (#19139)

[vitess-bot]

Description

This is a backport of #19139

[codecov]

Codecov Report

❌ Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 69.71%. Comparing base (7c037ab) to head (f9af58e).
⚠️ Report is 1 commits behind head on release-23.0.

Files with missing lines	Patch %	Lines
go/vt/vtgate/planbuilder/operators/union.go	88.88%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           release-23.0   #19162   +/-   ##
=============================================
  Coverage         69.71%   69.71%           
=============================================
  Files              1606     1606           
  Lines            214604   214599    -5     
=============================================
+ Hits             149606   149609    +3     
+ Misses            64998    64990    -8     

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