Site Migrations flow: On Sites dashboard, remove link to Checklist for sites in migration started or migration pending status

[sixhours]

Remove the link to the Checklist from the Sites dashboard for sites that are migration started or migration pending. We don't want users to use the checklist in these cases because the incoming migration will overwrite anything they might do.

[github-actions]

OpenAI suggested the following labels for this issue:

• [Feature Group] Site Migration: The issue directly pertains to the process and tools involved with site migrations, specifically modifications to the dashboard during migration status.
• [Feature] Site Migration: This label is relevant as it explicitly covers the features related to migrating sites to WordPress.com.
• [Feature] Site Settings: The issue addresses changes to the dashboard that affect how site settings are managed during the migration process.
• [Feature] My Home: The reference to the Sites dashboard suggests that it involves the main area where users manage their sites.