Bug Report: OnlineDDL cancelled migration artifacts are not getting cleaned up

[shlomi-noach]

Overview of the Issue

Followup to, and a bit of an oversight of #13927. Migrations that have been cancelled may have artifacts (specifically ALTER TABLE migrations will have artifacts if cancelled after starting).

The migration cleanup mechanism does not collect artifacts of cancelled migrations; only of completed and failed migrations, as per:

vitess/go/vt/vttablet/onlineddl/schema.go

Lines 343 to 354 in a8f601d

	sqlSelectUncollectedArtifacts = `SELECT
	migration_uuid,
	artifacts,
	log_path
	FROM _vt.schema_migrations
	WHERE
	migration_status IN ('complete', 'failed')
	AND cleanup_timestamp IS NULL
	AND completed_timestamp <= IF(retain_artifacts_seconds=0,
	NOW() - INTERVAL %a SECOND,
	NOW() - INTERVAL retain_artifacts_seconds SECOND
	)

#13928 fixed completed_timestamp to ensure it's not null, but that was not enough, we should also have updated sqlSelectUncollectedArtifacts.

Reproduction Steps

Cancel a vitess ALTER TABLE migration halfway through.

Binary Version

18, 17, 16, probably 15

Operating System and Environment details

-

Log Fragments

No response