Migrations: Update all `/setup/migration` entrypoints to use the new experiment name

[gabrielcaires]

Goal

We want to enable users to be part of the new [2] migration flow experiment, so we need to update all entry points pointing to /setup/migration to use the new experiment name.

Entrypoints:

• /move managed by our experiment blog
• /hosting managed by our experiment blog
• /sites > "Add new site` > "Import"

  wp-calypso/client/sites/hooks/use-sites-dashboard-import-site-url.ts

  Line 18 in 036325c

  experimentAssignment?.variationName === 'treatment'