test: Migrate e2e mock flow

[DafyddLlyr]

How I narrowed this down -

• Run regression tests locally (just the failing test)
• Checkout API docker logs
• Confirm this is a schema validation error
• Narrow down actual error using https://json-schema.app/view/%23?url=https%3A%2F%2Ftheopensystemslab.github.io%2Fdigital-planning-data-schemas%2Fv0.7.1%2Fschemas%2Fapplication.json and the generated session (visit http://localhost:7002/admin/session/<my-e2e-session-id>/digital-planning-application?skipValidation=true)
• Realise it's a migration issue
• Run dec2024 migration script on e2e db
• Copy/paste updated flow from local e2e db to mock json file
• Re-run tests locally ✅

✅ Regression tests passing here on CI - https://github.com/theopensystemslab/planx-new/actions/runs/12745837198

[github-actions]

Removed vultr server and associated DNS entries

[freemvmt]

Thanks for documenting your steps so clearly! ✨