Backfill spdx column in software_license_policies table

In MR !171584 (merged), we added the software_license_spdx_identifier column to the software_license_policies table.

In order to remove the software_license table we need to backfill the software_license_spdx_identifier column in software_license_policies table and updates our code to populated the software_license_spdx_identifier field when a new software license policy is created.