pg-cdc old syntax: Fix test failure

[def-]

Follow-up to #30807

Seen in https://buildkite.com/materialize/nightly/builds/10822#01944f9a-9738-4aa8-a012-bc111a2660ae

@petrosagg Is this expected? Feels like it could also be a bug since we can now still ingest a value after having dropped the column.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.