Resolve "Remove RestrictAllowedSchemas.with_suppressed from db/post_migrate/20240209153920_queue_purge_security_scans_with_empty_finding_data.rb"

What does this MR do and why?

While migrating tables to the new gitlab_sec database, we temporarily needed to suppress the restriction of allowed schema during certain migrations where not all tables had yet been transitioned.

This merge request removes the temporary impression of the schema restriction, as it is no longer necessary following the completion of the table migrations.

MR acceptance checklist

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Related to #477986 (closed)