BE - update data migration scripts + model for amalgamation

[argush3]

TODOs

• Do work in feature-legal-name branch
• Tweak amalgamation alembic script that was merged in from main branch. At least business_id will need to be updated to legal_entity_id
  - [ ] Update amalgamation model claases as required Done in Investigate how to merge missing updates in main into legal name branch #18452
• Update the database transfer script to include amalgamation model changes made in Legal API: model updates #18977
• Reference readme for details around setting up environment to run transfer script. The steps will be a bit different and Argus can provide a source LEAR db that reduces the amount of work to test the transfer script. Also part 2 in readme is not relevant to this work.
• Consult with Argus on the setup and approach when starting work.

[severinbeauvais]

I have untagged this ticket from release 20.2a because this ticket doesn't have any changes in this release.

Here are the only changes in 20.2a since the last release:
https://github.com/bcgov/lear/releases/tag/untagged-d521a6ec5f72377dfbfa