[release-21.0] schemadiff: skip keys with expressions in Online DDL analysis (#17475)

[vitess-bot]

Description

This is a backport of #17475

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 70.58824% with 5 lines in your changes missing coverage. Please review.

Project coverage is 67.53%. Comparing base (47883f1) to head (dfceec3).
Report is 3 commits behind head on release-21.0.

Files with missing lines	Patch %	Lines
go/vt/schemadiff/onlineddl.go	25.00%	3 Missing ⚠️
go/vt/schemadiff/key.go	66.66%	2 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release-21.0   #17480      +/-   ##
================================================
+ Coverage         67.51%   67.53%   +0.02%     
================================================
  Files              1570     1570              
  Lines            251442   251455      +13     
================================================
+ Hits             169756   169824      +68     
+ Misses            81686    81631      -55     

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