release-24.3: opt: fix trigger+computed column internal error

[blathers-crl]

Backport 1/1 commits from #154789 on behalf of @mgartner.

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This commit fixes a bug that was caused by incorrectly modifying the set
and list of target columns when re-projecting computed columns after
building a BEFORE trigger.

Fixes #154672

Release note (bug fix): A bug has been fixed that caused internal errors
for INSERT .. ON CONFLICT .. DO UPDATE statements when the target
table had both a computed column and a BEFORE trigger. This bug has
been present since triggers were introduced in v24.3.0.

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Release justification: Low-risk bug fix.

[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that it falls into one of the following categories (select one):

• Non-production code changes. Includes test-only changes, build system changes, etc.
• Fixes for serious issues. Defined in the policy as correctness, stability, or security issues, data corruption/loss, significant performance regressions, breaking working and widely used functionality, or an inability to detect and debug production issues.
• Other approved changes. These changes must be gated behind a disabled-by-default feature flag unless there is a strong justification not to.

Add a brief release justification to the PR description explaining your selection.

Also, confirm that the change does not break backward compatibility and complies with all aspects of the backport policy.

All backports must be reviewed by the TL and EM for the owning area.

[blathers-crl]

❌ PR #155074 does not comply with backport policy

Confidence: high
Explanation: The PR does not meet the critical bug requirement under the backport policy, nor does it include a disabled feature flag to gate the changes. Although a bug that causes internal errors is addressed, this does not categorically fall under the specified critical bug criteria such as stability/security issues, data corruption/loss, significant performance regressions, incorrect results/suboptimal performance, or inability to debug production issues. The PR description mentions a 'Low-risk bug fix' which doesn't align with critical issues. Moreover, there is no evidence of this change being gated behind a feature flag. No new cluster or session settings are introduced that control the behavior of new features or changes. The changes are in production files and not exempt non-production paths.
Recommendation: Gate the PR behind a feature flag or reconsider the backport. Add a justification as to why this bug fix is critical that allows safe backporting without a feature flag.

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[mgartner]

The other backports passed the AI check, so...