private Set<String> getAllFolderUuids() {
        final String sql = """
                select distinct uuid
                from explorer_node
                where type = 'Folder'""";
        return getUuids(explorerDbConnProvider, sql, "uuid", "folder");
    }
Previously deletion of a doc or proc_filter did not delete the doc_permission records for it.
This migration deletes any doc_permission records for a doc_uuid that cannot be found
in doc, processor_filter or explorer_node and is not the System doc (uuid == 0).
<p>
The migration copies the orphaned doc_permission records to a backup table
({@link V07_05_00_005__Orphaned_Doc_Perms#BACKUP_TBL_NAME}) before deleting them.
</p>
stroom-app/src/main/java/stroom/app/db/migration/V07_05_00_005__Orphaned_Doc_Perms.java