Don't force "before" to exist in Debezium

[umanwizard]

This was necessary pre-DEBEZIUM UPSERT, but isn't anymore.

[morsapaes]

Thanks for patching this, @umanwizard! I've pinned down documentation and integration testing dependencies. Leaving the approval to Petros and the Storage team.

[aljoscha]

Seems good! My only concern would be if we do want to use ENVELOPE DEBEZIUM for an efficient implementation in the future again. If we now relax the requirements of ENVELOPE DEBEZIUM we'd have to introduce a different name for that in the future.

Not sure that future will ever come, though ... 🤷‍♂️

[umanwizard]

You're right, and I hope that day does someday come! But FWIW, the plan has always been to leave DEBEZIUM UPSERT as the default, and make such a mode be enabled by special syntax or a WITH option. So I don't think this is a regression on that front.

[benesch]

I know I sound like a broken record, but: this needs a test and a release note.

[benesch]

Testing is probably somewhat tricky, but I think we can do it by configuring one of the Debezium PostgreSQL CDC tests to replicate from a table with REPLICA IDENTITY NOTHING.

[benesch]

I filed https://github.com/MaterializeInc/database-issues/issues/5580 to track the test.