release-24.3: workload/schemachanger: fix drop columns during PK swaps

[blathers-crl]

Backport 1/1 commits from #136738 on behalf of @fqazi.

/cc @cockroachdb/release

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Previously, if a declarative alter primary key and drop column were executed concurrently this workload did not correctly pick the correct columns. It was possible for the new primary key column to be selected for the drop operation. To address this, this patch allows potential execution errors for invalid column references when a PK swap is in progress.

Fixes: #134056
Fixes: #136661
Fixes: #136405
Fixes: #132298

Release note: None

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Release justification: test only change to deflake

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

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[cockroach-teamcity]

This change is 

[fqazi]

Also picked up: #136833