Feature/cls2 849 investment projects record multiple programmes part 3

[Richard-Pentecost]

Description of change

Update admin and serializers to allow user to update specific_programme from specific_programmes field and to make both fields behave the same whilst migrating from 'one to many' to a 'many to many' relationship

Checklist

• Has this branch been rebased on top of the current main branch?

  Explanation

  The branch should not be stale or have conflicts at the time reviews are requested.

• Is the CircleCI build passing?

General points

Other things to check

• Make sure fixtures/test_data.yaml is maintained when updating models
• Consider the admin site when making changes to models
• Use select-/prefetch-related field lists in views and search apps, and update them when fields are added
• Make sure the README is updated e.g. when adding new environment variables

See docs/CONTRIBUTING.md for more guidelines.