Context

This is the first of three PRs related to the new withdrawal reasons journey and data model. This first one is just the migration. The next two will be (1) Feature flag, models, yml files (2) UI journey

Changes proposed in this pull request

No UI changes, just the migration.

Guidance to review

Are we happy with the table name 'withdrawal_reasons?'

Things to check

• If the code removes any existing feature flags, a data migration has also been added to delete the entry from the database
• This code does not rely on migrations in the same Pull Request
• If this code includes a migration adding or changing columns, it also backfills existing records for consistency
• If this code adds a column to the DB, decide whether it needs to be in analytics yml file or analytics blocklist, if included inform data insights team of the changes
• If this code adds a column that may include PII, the sanitise.sql script and 0025-protecting-personal-data-in-production-dump.md ADR have been updated.
• API release notes have been updated if necessary
• If it adds a significant user-facing change, is it documented in the CHANGELOG?
• Attach the PR to the Trello card